THREAD

Before this fix, two rows in table performance_schema.threads
could be seen with the same THREAD_OS_ID.

The root cause is the instrumentation for innodb thread
srv_purge_coordinator_thread, which created instrumentation twice:
- once in the call to create_thd(),
- once in the call to pfs_register_thread()

Also, every background threads created by create_thd() were instrumented
with a generic "thread/sql/background", which provides no valuable
information.

The fix is to:
- remove the "thread/sql/background" instrument entirely,
- pass a proper thread instrumentation key to create_thd(),
- remove the calls to pfs_register_thread() when they are duplicates.