Before this change, a client could spend funds before the accountant
processed a previous spend. With this change in place, the accountant
updates balances immediately, but that comes at an architectural cost.
The accountant now verifies signatures on behalf of the historian, so
that it can ensure logging will not fail.
The log crate was starting to be the catch-all for all things
related to entries, events, signatures, and hashes. This split
shows us that:
* Event depends only on signatures, not on hashes [directly]
* All event testing was done via log testing (shame on me)
* Accounting depends only on events
Greg Fitzgerald