wait events for disk I/O

Hi All,

Attached is the patch, which extend the existing wait event infrastructure
to
implement the wait events for the disk I/O. Basically pg_stat_activity's
wait
event information to show data about disk I/O as well as IPC primitives.

Implementation details:

- Added PG_WAIT_IO to pgstat.h and a new enum WaitEventIO
- Added a wait_event_info argument to FileRead, FileWrite, FilePrefetch,
FileWriteback, FileSync, and FileTruncate. Set this wait event just before
performing the file system operation and clear it just after.
- Pass down an appropriate wait event from caller of any of those
functions.
- Also set and clear a wait event around standalone calls to read(),
write(), fsync() in other parts of the system.
- Added documentation for all newly added wait event.

Open issue:
- Might missed few standalone calls to read(), write(), etc which need
to pass the wait_event_info.

Thanks to my colleague Robert Haas for his help in design.

Please let me know your thought, and thanks for reading.

Thanks,
Rushabh Lathia
www.EnterpriseDB.com