leveldb: compaction: Implements rollback for compaction transactions
storage: MemStorage: Allows file with same number but different type to coexist
leveldb: storage_test: Allows filesystem-backed storage used for testing

Use GOLEVELDB_USEFS=1 to enable filesystem-backed storage or GOLEVELDB_USEFS=2
to also prevent the test storage to be removed when test is done.
storage: FileStorage: Also scan pending CURRENT files