Implemented RocksDB::WriteOptions

Summary: Setting WriteOptions through global system variables. The options are read-write. The WriteOptions object is copied for each use to avoid threading issues.

Test Plan:
Ran the test. Observed that sync option turns on and off WAL flushes per transaction (ie SQL statement). Stopped in the gdb and checked that WriteOptions had correct values.

The diff is for this task: https://github.com/MariaDB/webscalesql-5.6/issues/15

Reviewers: mcallaghan, jonah

Subscribers: webscalesql-eng@

Differential Revision: https://phabricator.fb.com/D1745210