Before this patch, the InnoDB purge coordinator task submitted
innodb_purge_threads-1 tasks even if there was not sufficient amount
of work for all of them. For example, if there are undo log records
only for 1 table, only 1 task can be employed, and that task had better
be the purge coordinator.

srv_purge_worker_task_low(): Split from purge_worker_callback().

trx_purge_attach_undo_recs(): Remove the parameter n_purge_threads,
and add the parameter n_work_items, to keep track of the amount of
work.

trx_purge(): Launch purge worker tasks only if necessary. The work of
one thread will be executed by this purge coordinator thread.

que_fork_scheduler_round_robin(): Merged to trx_purge().

Thanks to Vladislav Vaintroub for supplying a prototype of this.

Reviewed by: Debarun Banerjee