logd: join() the SerializedLogBuffer deleter thread

Logd never deletes SerializedLogBuffer, so it seemed reasonable to
detach the deleter thread, however unit tests and fuzzers do delete
SerializedLogBuffer, so we must safely join the deleter thread in the
destructor.

This simplifies the deleter thread code and ensures that only one
deleter thread will be running at a time.

Test: fuzzing works
Change-Id: I69c7447109898a1bb7038a03337cadacb1213281