The transaction_monitor function in WalletService was polling the most
recent 100 transactions from the BlockchainInterface every 5 seconds.
This was egregious and inefficient. Introduce a _yield_new_transactions
function that remembers the newest previously yielded transaction
between invocations and yields only transactions newer than that one.
Use the new function in transaction_monitor.
Avoid repeatedly deserializing confirmed transactions that remain in the
set of monitored transactions after being confirmed either due to
"confirmed" callbacks that return False or because callbacks have timed
out and have been unregistered without cleaning the set of monitored
transactions. Change the active_txids list to an active_txs dict whose
keys are the txids and whose values are the deserialized transactions.
Use this new dict as a cache to avoid repeatedly deserializing confirmed
transactions.
Matt Whitlock
f52bf71740