Remove rocks_db import from persist() function

why:
  Some systems (i.p `fluffy` and friends) use the `Aristo` memory
  backend emulation and do not link against rocksdb when building the
  application. So this should fix that problem.

nimbus/db/kvt/kvt_desc.nim

@@ -42,7 +42,7 @@ type
     centre: KvtDbRef                  ## Link to peer with write permission
     peers: HashSet[KvtDbRef]          ## List of all peers
 
-  KvtDbRef* = ref object
+  KvtDbRef* = ref object of RootRef
     ## Three tier database object supporting distributed instances.
     top*: LayerRef                    ## Database working layer, mutable
     stack*: seq[LayerRef]             ## Stashed immutable parent layers

nimbus/db/kvt/kvt_desc/desc_backend.nim

@@ -59,6 +59,15 @@ type
       ## `KVT` descriptors being modified (e.g. by `reCentre()`) or by
       ## adding/removing a new peer (e.g. by `fork()` or `forget()`.)
 
+  SetWrReqFn* =
+    proc(db: RootRef): Result[void,KvtError] {.gcsafe, raises: [].}
+      ## This function stores a request function for the piggiback mode
+      ## writing to the `Aristo` set of column families.
+      ##
+      ## If used at all, this function would run `rocks_db.setWrReqTriggeredFn()()`
+      ## with a `KvtDbRef` type argument for `db`. This allows to run the `Kvt`
+      ## without linking to the rocksdb interface unless it is really needed.
+
   # -------------
 
   BackendRef* = ref BackendObj
@@ -74,6 +83,8 @@ type
     closeFn*: CloseFn                ## Generic destructor
     canModFn*: CanModFn              ## Lock-alike
 
+    setWrReqFn*: SetWrReqFn          ## Register main descr for write request
+
 proc init*(trg: var BackendObj; src: BackendObj) =
   trg.getKvpFn = src.getKvpFn
   trg.putBegFn = src.putBegFn

nimbus/db/kvt/kvt_desc/desc_error.nim

@@ -28,6 +28,7 @@ type
     RdbBeDriverPutError
     RdbBeDriverWriteError
     RdbBeHostError
+    RdbBeHostNotApplicable
 
     # Transaction wrappers
     TxArgStaleTx

nimbus/db/kvt/kvt_init/memory_db.nim

@@ -125,6 +125,11 @@ proc canModFn(db: MemBackendRef): CanModFn =
     proc(): Result[void,KvtError] =
       ok()
 
+proc setWrReqFn(db: MemBackendRef): SetWrReqFn =
+  result =
+    proc(kvt: RootRef): Result[void,KvtError] =
+      err(RdbBeHostNotApplicable)
+
 # ------------------------------------------------------------------------------
 # Public functions
 # ------------------------------------------------------------------------------
@@ -142,7 +147,7 @@ proc memoryBackend*: BackendRef =
 
   db.closeFn = closeFn db
   db.canModFn = canModFn db
-
+  db.setWrReqFn = setWrReqFn db
   db
 
 proc dup*(db: MemBackendRef): MemBackendRef =

nimbus/db/kvt/kvt_init/rocks_db.nim

@@ -142,6 +142,11 @@ proc canModFn(db: RdbBackendRef): CanModFn =
     proc(): Result[void,KvtError] =
       ok()
 
+proc setWrReqFn(db: RdbBackendRef): SetWrReqFn =
+  result =
+    proc(kvt: RootRef): Result[void,KvtError] =
+      err(RdbBeHostNotApplicable)
+
 # ------------------------------------------------------------------------------
 # Private functions: triggered interface changes
 # ------------------------------------------------------------------------------
@@ -194,6 +199,15 @@ proc canModTriggeredFn(db: RdbBackendRef): CanModFn =
         return err(RdbBeDelayedLocked)
       ok()
 
+proc setWrReqTriggeredFn(db: RdbBackendRef): SetWrReqFn =
+  result =
+    proc(kvt: RootRef): Result[void,KvtError] =
+      if db.rdb.delayedPersist.isNil:
+        db.rdb.delayedPersist = KvtDbRef(kvt)
+        ok()
+      else:
+        err(RdbBeDelayedAlreadyRegistered)
+
 # ------------------------------------------------------------------------------
 # Private function: trigger handler
 # ------------------------------------------------------------------------------
@@ -260,6 +274,7 @@ proc rocksDbKvtBackend*(
 
   db.closeFn = closeFn db
   db.canModFn = canModFn db
+  db.setWrReqFn = setWrReqFn db
   ok db
 
 
@@ -288,21 +303,10 @@ proc rocksDbKvtTriggeredBackend*(
 
   db.closeFn = closeTriggeredFn db
   db.canModFn = canModTriggeredFn db
+  db.setWrReqFn = setWrReqTriggeredFn db
   ok db
 
 
-proc setDelayedPersist*(
-    be: BackendRef;
-    kvt: KvtDbRef;
-      ): Result[void,KvtError] =
-  let db = RdbBackendRef(be)
-  if db.rdb.delayedPersist.isNil:
-    db.rdb.delayedPersist = kvt
-    ok()
-  else:
-    err(RdbBeDelayedAlreadyRegistered)
-
-
 proc dup*(db: RdbBackendRef): RdbBackendRef =
   new result
   init_common.init(result[], db[])

nimbus/db/kvt/kvt_tx.nim

@@ -16,7 +16,7 @@
 import
   results,
   ./kvt_tx/[tx_fork, tx_frame, tx_stow],
-  ./kvt_init/[memory_only, rocks_db],
+  ./kvt_init/memory_only,
   ./kvt_desc
 
 # ------------------------------------------------------------------------------
@@ -181,7 +181,7 @@ proc persist*(
   # Register for saving if piggybacked on remote database
   if db.backend.kind == BackendRdbTriggered:
     ? db.txStowOk(persistent=true)
-    ? db.backend.setDelayedPersist db
+    ? db.backend.setWrReqFn db
     return err(TxPersistDelayed)
 
   db.txStow(persistent=true)