Implement server side changes for XtraBackup MyRocks

[george-lorch]

• Requires RocksDB submodule to be up to or past 3f5c1236026233b34be64d597fe660d7e698f2f5

• Isolated all MyRocks 8 code chenges within #ifdef MYROCKS_8. Once this code is unblocked, rocksdb.rocksdb will need to be re-recorded.

• Added new session variable to MyRocks rocksdb_create_temporary_checkpoint which will create a temporary RocksDB 'checkpoint' or 'snapshot'. If the session ends with an existing checkpoint, or if the variable is reset to another value, the checkpoint will get removed.

• Added new session variable to MyRocks rocksdb_disable_file_deletions, which when set, will tell RocksDB to disable the deletion of old WAL and .sst files. If the session ends without re-enabling file deletion, file deletions will be explicitly re-enabled.

• Expose rocksdb WAL position in i_s.log_status table.

• Added tests to MyRocks for new options and behaviors.