MDEV-232: Remove one fsync() from commit phase.

Introduce a new storage engine API method commit_checkpoint_request().
This is used to replace the fsync() at the end of every storage engine
commit with a single fsync() when a binlog is rotated.

Binlog rotation is now done during group commit instead of being
delayed until unlog(), removing some server stall and avoiding an
expensive lock/unlock of LOCK_log inside unlog().

mysql-test/extra/rpl_tests/rpl_insert_delayed.test

@@ -133,7 +133,7 @@ if  (`SELECT @@global.binlog_format = 'STATEMENT'`)
 {
   #must show two INSERT DELAYED
   --let $binlog_file= query_get_value(SHOW MASTER STATUS, File, 1)
-  --let $binlog_limit= 1,6
+  --let $binlog_limit= 2,6
   --source include/show_binlog_events.inc
 }
 select * from t1;