Move all kill mutex protection to LOCK_thd_kill

LOCK_thd_data was used to protect both THD data and
ensure that the THD is not deleted while it was in use

This patch moves the THD delete protection to LOCK_thd_kill,
which already protects the THD for kill.

The benefits are:
- More well defined what LOCK_thd_data protects
- LOCK_thd_data usage is now much simpler and easier to verify
- Less chance of deadlocks in SHOW PROCESS LIST as there is less
  chance of interactions between mutexes
- Remove not needed LOCK_thread_count from
  thd_get_error_context_description()
- Fewer mutex taken for thd->awake()

Other things:
- Don't take mysys->var mutex in show processlist to check if thread
  is kill marked
- thd->awake() now automatically takes the LOCK_thd_kill mutex
  (Simplifies code)
- Apc uses LOCK_thd_kill instead of LOCK_thd_data

mysql-test/suite/perfschema/t/nesting.test

@@ -39,6 +39,7 @@ update performance_schema.setup_instruments set enabled='YES', timed='YES'
                  'wait/io/socket/sql/client_connection',
                  'wait/synch/rwlock/sql/LOCK_grant',
                  'wait/synch/mutex/sql/THD::LOCK_thd_data',
+                 'wait/synch/mutex/sql/THD::LOCK_thd_kill',
                  'wait/io/file/sql/query_log');
 
 update performance_schema.setup_instruments set enabled='YES', timed='YES'