purge_sys_t::wake_if_not_active(): Replaces
srv_wake_purge_thread_if_not_active().

innodb_ddl_recovery_done(): Move the wakeup call to
srv_init_purge_tasks().

purge_coordinator_timer: Remove. The srv_master_callback() already
invokes purge_sys.wake_if_not_active() once per second.

Reviewed by: Vladislav Lesin and Vladislav Vaintroub