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/suite/innodb/r/group_commit_binlog_pos.result

@@ -1,3 +1,4 @@
+SET GLOBAL innodb_flush_log_at_trx_commit=3;
 CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=innodb;
 INSERT INTO t1 VALUES (0);
 SET DEBUG_SYNC= "commit_after_get_LOCK_log SIGNAL con1_waiting WAIT_FOR con3_queued";