- KVDB/StateDB
        - Pass config parameters in a Config type instead of using many
          arguments in constructor.
	- Add new parameter `NoLast` which disables having an opened DB with a
	  checkpoint to the last batchNum for thread-safe reads.  Last will be
	  disabled in the StateDB used by the TxSelector and BatchBuilder.
	- Add new parameter `NoGapsCheck` which skips checking gaps in the list
	  of checkpoints and returning errors if there are gaps.  Gaps check
	  will be disabled in the StateDB used by the TxSelector and
	  BatchBuilder, because we expect to have gaps when there are multiple
	  coordinators forging (slots not forged by our coordinator will leave
	  gaps).