Introduce new transaction store.

This change replaces the old transaction store file format and
implementation.  The most important change is how the full backing
transactions for any received or sent transaction are now saved,
rather than simply saving parsed-out details of the tx (tx shas, block
height/hash, pkScripts, etc.).

To support the change, notifications for received transaction outputs
and txs spending watched outpoints have been updated to use the new
redeemingtx and recvtx notifications as these contain the full tx,
which is deserializead and inserted into the store.

The old transaction store serialization code is completely removed, as
updating to the new format automatically cannot be done.  Old wallets
first running past this change will error reading the file and start a
full rescan to rebuild the data.  Unlike previous rescan code,
transactions spending outpoint managed by wallet are also included.
This results in recovering not just received history, but history for
sent transactions as well.