Cache should decide when to commit on its own

**Is your feature request related to a problem? Please describe.**
Now cache still commits upon receiving `Commit` message from `EpochManager`, but the pipeline has nothing to commit and the `EpochManager` should be removed.

The indexing threads only commits when indexing is done, which is not good.

**Describe the solution you'd like**
Cache main db and indexing db generates decide when to commit on its own using number of operations processed and timeout.