Add more testing

[ecton]

When revisiting this project after a bit of a break, I ran through code coverage to look to see what was missing. These are areas that I didn't already add tests for:

• checkpoint target tests
• Database PartialEq
• multithreaded CommitLog reading (CommitLogCacheEntry::Caching waiting)
• allocations: https://khonsulabs.github.io/sediment/coverage/src/allocations.rs.html