This adds a Flush method to the DB interface so that users of the DB
type can flush the underlying database to disk as needed.

The immediate use case for this is to allow for flushing dependencies.
For example, the block database always needs to be flushed to disk prior
to the UTXO database being flushed to disk to ensure that the UTXO
database remains in a recoverable state in the event of an unclean
shutdown.

This adds a flushBlockDB function to the UTXO cache.  The flushBlockDB
function is used to flush the block database to disk prior to flushing
the UTXO cache to the UTXO database.

This ensures that the block database is always at least as far along as
the UTXO database which keeps the UTXO database in a recoverable state
in the event of an unclean shutdown.

This forces the UTXO database to flush to disk after initializing the
UTXO set state for the first time.  This is necessary so that if the
block database is flushed, and then an unclean shutdown occurs, the UTXO
cache will know where to start from when recovering on startup.

This modifies the separateUtxoDatabase upgrade to force the UTXO
database to flush to disk prior to removing the UTXO set and state from
the block database.

This prevents a scenario where the UTXO set is removed from the block
database and the block database is flushed to disk, but an unclean
shutdown occurs before the UTXO database flushes to disk, leaving the
UTXO database in an unrecoverable state.