Show thread pool wait status in SHOW PROCESSLIST

Currently a connection that has issued a query, but which is still in the pool-of-threads wait queue waiting to be executed, just shows with status "Sleep" and no query text, just as if the client was idle and had not sent any query at all, and the time info does not even reflect the time at which the waiting query was sent to the server, but the time the previous query ended (which also leads to MDEV-21101).

Features request:

• add a query state like "Waiting"
• let the time column show the actual "Time" the query so far spent waiting in the queue
• show actual query in "Info"
• optionally: show the queue position, to get an idea when the query might eventually get executed