Optimize iteration over PGPROC for fast-path lock searches.

Optimize iteration over PGPROC for fast-path lock searches.

This commit improves efficiency in FastPathTransferRelationLocks()
and GetLockConflicts(), which iterate over PGPROCs to search for
fast-path locks.

Previously, these functions recalculated the fast-path group during
every loop iteration, even though it remained constant. This update
optimizes the process by calculating the group once and reusing it
throughout the loop.

The functions also now skip empty fast-path groups, avoiding
unnecessary scans of their slots. Additionally, groups belonging to
inactive backends (with pid=0) are always empty, so checking
the group is sufficient to bypass these backends, further enhancing
performance.

Author: Fujii Masao 
Reviewed-by: Heikki Linnakangas 
Reviewed-by: Ashutosh Bapat 
Discussion: https://postgr.es/m/07d5fd6a-71f1-4ce8-8602-4cc6883f4bd1@oss.nttdata.com