Refactor Message type to remove deletedAt and add updateSerial

Added an updateSerial to allow LWW semantics to be applied to a message resulting from an operation.
Removed the deletedAt timestamp, we will rely only on updateSerial to apply ordering of operations.
UpdateAt will still be used, but just so clients can render a simple datetime and not have to understand/parse the serial.

ably.d.ts

@@ -2356,9 +2356,9 @@ export interface Message {
    */
   updatedAt?: number;
   /**
-   * If a `deletion` operation was applied to this message, this will be the timestamp the deletion occurred.
+   * The serial of the operation that updated this message.
    */
-  deletedAt?: number;
+  updateSerial?: string;
   /**
    * If this message resulted from an operation, this will contain the operation details.
    */

src/common/lib/types/message.ts

@@ -341,7 +341,7 @@ class Message {
   refSerial?: string;
   refType?: string;
   updatedAt?: number;
-  deletedAt?: number;
+  updateSerial?: string;
   operation?: API.Operation;
 
   /**
@@ -378,7 +378,7 @@ class Message {
       refSerial: this.refSerial,
       refType: this.refType,
       updatedAt: this.updatedAt,
-      deletedAt: this.deletedAt,
+      updateSerial: this.updateSerial,
       operation: this.operation,
       encoding,
       data,
@@ -407,7 +407,7 @@ class Message {
     if (this.refSerial) result += '; refSerial=' + this.refSerial;
     if (this.refType) result += '; refType=' + this.refType;
     if (this.updatedAt) result += '; updatedAt=' + this.updatedAt;
-    if (this.deletedAt) result += '; deletedAt=' + this.deletedAt;
+    if (this.updateSerial) result += '; updateSerial=' + this.updateSerial;
     if (this.operation) result += '; operation=' + JSON.stringify(this.operation);
     result += ']';
     return result;