Optimize NotifyNewTxListener.

Looking up transactions from the database is an expensive operation.
This commit modifies the NotifyNewTxListener code to simply iterate the
transactions in the block instead of looking them up from the db.

Currently the wallet code needs a spent flag which ultimately shouldn't be
required.  For now, the spent data is simply created on the fly which is
still significantly faster than doing database transaction lookups.

Closes #24.