MDEV-5289: master server starts slave parallel threads

Delay spawning parallel replication worker threads until a slave SQL
thread is running, and de-spawn them when the last SQL thread stops.

This is especially useful to avoid needless threads on a master in a
setup where same my.cnf is used on masters and slaves.

mysql-test/suite/sys_vars/t/slave_parallel_threads_basic.cnf

@@ -0,0 +1,5 @@
+# Use default setting for mysqld processes
+!include include/default_mysqld.cnf
+
+[mysqld.1]
+slave_parallel_threads=20