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 https://github.com/facebook/rocksdb/issues/6112

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8114

Reviewed By: akankshamahajan15

Differential Revision: D27362923

Pulled By: anand1976

fbshipit-source-id: e93a8b1ffe26dafbe22529907f72b796ae971214