Add basic transaction store logging.

The info log level (default) will produce output about confirmed and
unconfirmed transactions being inserted into the store, as well as
unconfirmed transactions which have been mined into blocks.  By
enabling the debug log level (-d TXST=debug), additional information
about transaction inputs and outputs is logged.  This includes the
total amount of previously-unspent outputs which have been marked
spent by the inserted transaction, and the output indexes and amounts
for each spendable output.  Additionally, the debug log level will log
whenever transactions are removed due to being a double spend of
another inserted transaction.