waddrmgr/scoped_manager: add nextAddresses cache update to db tx's OnCommit()
waddrmgr/scoped_manager: add nextAddresses cache update to db tx's OnCommit()

This commit makes nextAddresses add a function to the transactions
OnCommit handler used to update the cache on successful database
transaction commit. Before this we would risk the cache and database of
get out of sync if the database transaction failed or was aborted after
the cache was updated.

btcsuite/btcwallet,btcsuite/btcwallet