Refactor GetLockStatusData() to skip backends/groups without fast-path locks.

Refactor GetLockStatusData() to skip backends/groups without fast-path locks.

Previously, GetLockStatusData() checked all slots for every backend
to gather fast-path lock data, which could be inefficient. This commit
refactors it by skipping backends with PID=0 (since they don't hold
fast-path locks) and skipping groups with no registered fast-path locks,
improving efficiency.

This refactoring is particularly beneficial, for example when
max_connections and max_locks_per_transaction are set high,
as it reduces unnecessary checks across numerous slots.

Author: Fujii Masao
Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/a0a00c44-31e9-4c67-9846-fb9636213ac9@oss.nttdata.com