Move deletion of old GTID rows to slave background thread

This patch changes how old rows in mysql.gtid_slave_pos* tables are deleted.
Instead of doing it as part of every replicated transaction in
record_gtid(), it is done periodically (every @@gtid_cleanup_batch_size
transaction) in the slave background thread.

This removes the deletion step from the replication process in SQL or worker
threads, which could speed up replication with many small transactions. It
also decreases contention on the global mutex LOCK_slave_state. And it
simplifies the logic, eg. when a replicated transaction fails after having
deleted old rows.

With this patch, the deletion of old GTID rows happens asynchroneously and
slightly non-deterministic. Thus the number of old rows in
mysql.gtid_slave_pos can temporarily exceed @@gtid_cleanup_batch_size. But
all old rows will be deleted eventually after sufficiently many new GTIDs
have been replicated.

mysql-test/main/mysqld--help.result

@@ -294,6 +294,15 @@ The following specify which files/extra groups are read (specified before remain
  --group-concat-max-len=# 
  The maximum length of the result of function
  GROUP_CONCAT()
+ --gtid-cleanup-batch-size=# 
+ Normally does not need tuning. How many old rows must
+ accumulate in the mysql.gtid_slave_pos table before a
+ background job will be run to delete them. Can be
+ increased to reduce number of commits if using many
+ different engines with --gtid_pos_auto_engines, or to
+ reduce CPU overhead if using a huge number of different
+ gtid_domain_ids. Can be decreased to reduce number of old
+ rows in the table.
  --gtid-domain-id=#  Used with global transaction ID to identify logically
  independent replication streams. When events can
  propagate through multiple parallel paths (for example
@@ -1434,6 +1443,7 @@ gdb FALSE
 general-log FALSE
 getopt-prefix-matching FALSE
 group-concat-max-len 1048576
+gtid-cleanup-batch-size 64
 gtid-domain-id 0
 gtid-ignore-duplicates FALSE
 gtid-pos-auto-engines