kvdb-rocksdb overlay cache is not effective

[NikVolf]

Seems that cache in kvdb itself (memorizing raw key-value pairs on write), despite showing effectiveness in synthetic benchmarks, offers no performance advantage for sync speed, sometimes even negative (anybody can try run polkadot it with parity-common replaced from this branch https://github.com/paritytech/parity-common/tree/no-overlay). Not counting increased memory footprint.

This is probably due to:

• There are a higher-level cache-s with good hit ratio.
• RocksDb has its own raw key-value cache, and quite configurable.

Given that, and also the fact that this cache prevents other possible optimisations (such as zero(little)-copy database transactions), we might want to remove it.

There are probably considerations other than sync speed, which would be great to hear about.

[NikVolf]

In fact, it is not used at all probably

So I might draft a version of kvdb with no overlay cache + significantly reduced copy of transaction data (now any key and any value must be put into dedicated Vec<u8>, and this totally can be avoided)

[ordian]

This resolved in paritytech/parity-common#313 I believe.