Add a new thread status for the query cache mutex

[Baron Schwartz]

One of the reasons people don't realize that the query cache is causing them serious trouble is that they don't see a SHOW PROCESSLIST full of queries in status "Waiting on query cache mutex." As long as we're building the server with the query cache enabled, I believe we should at least ship it with a way to see if something is waiting for the mutex. I suggest setting thd->status to "Waiting on query cache mutex" just before all operations on that mutex. Then it will be blindingly obvious that the query cache is the problem, and people can solve their own performance issues instead of having to hire expert help.

[Baron Schwartz (baron-xaprb)]

I don't mean Status, I mean State. Sorry ;-( So, I'd like to see something like this:

*************************** 1. row ***************************
     Id: 16838
   User: webuser
   Host: 10.10.16.82
     db: website
Command: Query
   Time: 0
  State: Waiting on query cache mutex
   Info: SELECT id FROM users WHERE username="john"

[Oleg Tsarev (tsarev)]

I need debug build for complete this feature. After fix source code for debug build, i wiil complete.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-411