This commit fixes a bug in the
TestDecayedLogPersistentGarbageCollector unit test.
The test generates a second hash prefix, which is never
added to the log, and used to query for the final
existence check. This commit reverts the behavior so
that the same hash prefix is used throughout the test.
This commit changes the decayed log tests to create
a new temporary database for each test. Previously, all
instances referenced the same db path. Since the tests
are run in parallel, the tests would create/delete the
shared db out from under each other, causing flakes in
the unit tests.
