RocksDB: remove expiration buckets

The expiration DB stores keys grouped by their expiration timestamp, in milliseconds. The chances of collision are pretty low, but having expiration information grouped by timestamp requires us to read the existing "bucket" every time we store an entry with expiration information.

Changing the format of the expiration DB to key = <expiration timestamp> + <key bytes>, value = <nothing> would allow us to assume that the expiration key is unique.