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: MariaDB/webscalesql-5.6#15

Reviewers: mcallaghan, jonah

mysql-test/suite/rocksdb/write_sync.result

@@ -0,0 +1,35 @@
+SET GLOBAL rocksdb_write_disable_wal=false;
+SET GLOBAL rocksdb_write_timeout_hint_us=555;
+SET GLOBAL rocksdb_write_ignore_missing_column_families=true;
+create table aaa (id int primary key, i int) engine rocksdb;
+SET GLOBAL rocksdb_write_sync=off;
+insert aaa(id, i) values(1,1);
+show status like 'rocksdb_wal_synced';
+Variable_name	Value
+rocksdb_wal_synced	1
+insert aaa(id, i) values(2,1);
+show status like 'rocksdb_wal_synced';
+Variable_name	Value
+rocksdb_wal_synced	1
+insert aaa(id, i) values(3,1);
+show status like 'rocksdb_wal_synced';
+Variable_name	Value
+rocksdb_wal_synced	1
+SET GLOBAL rocksdb_write_sync=on_commit;
+insert aaa(id, i) values(4,1);
+show status like 'rocksdb_wal_synced';
+Variable_name	Value
+rocksdb_wal_synced	2
+insert aaa(id, i) values(5,1);
+show status like 'rocksdb_wal_synced';
+Variable_name	Value
+rocksdb_wal_synced	3
+insert aaa(id, i) values(6,1);
+show status like 'rocksdb_wal_synced';
+Variable_name	Value
+rocksdb_wal_synced	4
+drop table aaa;
+SET GLOBAL rocksdb_write_sync=off;
+SET GLOBAL rocksdb_write_disable_wal=false;
+SET GLOBAL rocksdb_write_timeout_hint_us=0;
+SET GLOBAL rocksdb_write_ignore_missing_column_families=false;

mysql-test/suite/rocksdb/write_sync.test

@@ -0,0 +1,29 @@
+SET GLOBAL rocksdb_write_disable_wal=false;
+SET GLOBAL rocksdb_write_timeout_hint_us=555;
+SET GLOBAL rocksdb_write_ignore_missing_column_families=true;
+
+create table aaa (id int primary key, i int) engine rocksdb;
+
+SET GLOBAL rocksdb_write_sync=off;
+insert aaa(id, i) values(1,1);
+show status like 'rocksdb_wal_synced';
+insert aaa(id, i) values(2,1);
+show status like 'rocksdb_wal_synced';
+insert aaa(id, i) values(3,1);
+show status like 'rocksdb_wal_synced';
+
+SET GLOBAL rocksdb_write_sync=on_commit;
+insert aaa(id, i) values(4,1);
+show status like 'rocksdb_wal_synced';
+insert aaa(id, i) values(5,1);
+show status like 'rocksdb_wal_synced';
+insert aaa(id, i) values(6,1);
+show status like 'rocksdb_wal_synced';
+
+# Cleanup
+drop table aaa;
+SET GLOBAL rocksdb_write_sync=off;
+SET GLOBAL rocksdb_write_disable_wal=false;
+SET GLOBAL rocksdb_write_timeout_hint_us=0;
+SET GLOBAL rocksdb_write_ignore_missing_column_families=false;
+