Summary:
The snapshot structure returned by rocksdb_transaction_get_snapshot is
supposed to be freed by calling rocksdb_free(), so allocate using malloc
rather than new. Fixes #6112

Pull Request resolved: #8114

Reviewed By: akankshamahajan15

Differential Revision: D27362923

Pulled By: anand1976

fbshipit-source-id: e93a8b1ffe26dafbe22529907f72b796ae971214