track last_{ach,bill}_result over time

[chadwhitacre]

It was a nice design decision to track tips over time, that is, to insert new records into the tips table rather than simply updating an existing record. The historical information is interesting and valuable for debugging. We should change last_bill_result and last_ach_result to use this pattern as well. This is especially important for investigating abuse (see #329).

See also goal (#141) and statement (#343).

[chadwhitacre]

Closing in favor of #1549.