Add recovery for utxo db

[yangl1996]

We now disable WAL for UTXODB, so should the process crash UTXODB will be in an inconsistent state. The solution is to manually flush the DB, and mark the state when we flush.

[wgr523]

I have two questions:

1. when do we want to flush?
2. can we ensure there is no flush other than the manual flush? since we can record the state when manual flush, but automatic flush will not record the state