memdb: Introduce DB.State method
leveldb: DB.recoverJournal: Allows reuse memdb
table: Writer: Also reset the data block prevKey after pending block handle flushed; just to make sure
leveldb: testing: Make FS-backed storage as the default backend for testStorage
storage: Bring back ReadAt
leveldb: testing: Add TestDb_SingleEntryMemCompaction test
leveldb: testing: Remove TestDb_ModifyingBuffer test

This test is irrelevant since the returned slice are not copied anymore.
iterator: mergedIterator: Assert nil key