Step1:
=====
Backport 'replication_applier_status_by_worker' from upstream.

Iterate through rpl_parallel_thread_pool and display slave worker thread
specific information as part of 'replication_applier_status_by_worker'
table.

---------------------------------------------------------------------------
|Column Name:           |        Description:                             |
|-------------------------------------------------------------------------|
|                       |                                                 |
|CHANNEL_NAME           | Name of replication channel through which the   |
|                       | transaction is received.                        |
|                       |                                                 |
|THREAD_ID              | Thread_Id as displayed in 'performance_schema.  |
|                       | threads' table for thread with name             |
|                       | 'thread/sql/rpl_parallel_thread'                |
|                       |                                                 |
|                       | THREAD_ID will be NULL when worker threads are  |
|                       | stopped due to an error/force stop              |
|                       |                                                 |
|SERVICE_STATE          | Thread is running or not                        |
|                       |                                                 |
|LAST_SEEN_TRANSACTION  | Last GTID executed by worker                    |
|                       |                                                 |
|LAST_ERROR_NUMBER      | Last Error that occured on a particular worker  |
|                       |                                                 |
|LAST_ERROR_MESSAGE     | Last error specific message                     |
|                       |                                                 |
|LAST_ERROR_TIMESTAMP   | Time stamp of last error                        |
|                       |                                                 |
---------------------------------------------------------------------------

CHANNEL_NAME will be empty when the worker has not processed any
transaction. Channel_name points to valid source channel_name when it is
processing a transaction/event group.