Sync wallet, utxo, and tx files to disk.

This runs a syncer once every minute to write any dirty wallet data
structures out to disk.  As currently implemented, dirty wallets will
be lost if not written before btcwallet closes or crashes.
Deterministic wallet help migitate this issue (as private keys can be
created again as long as a previous wallet file was written) but this
can still be a nuisance as a longer rescan will be required to catch
up to chain.