wait event and archive_command

Hi,

I'd like to propose to add new wait event reported while archiver process
is executing archive_command. This would be helpful to observe
what archiver is doing and check whether it has some troubles or not.
Thought? PoC patch attached.

Also how about adding wait events for other commands like
archive_cleanup_command, restore_command and recovery_end_command?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION