mon/MgrStatMonitor.cc: cleanup pool_availability

mon/MgrStatMonitor.cc: cleanup pool_availability

This commit does the following:
1. In `calc_pool_availability()`, instead of swapping
pool_availability and pending_pool_availability (which resulted in
pool_availability storing old object value and not the latest object
values), we now simply update pending_pool_availability to the
newest calculated values.

2. `calc_pool_availability` now updates pool_availability only in
leader mon. For peon, it will simply exit early.

Fixes: https://tracker.ceph.com/issues/71857
Signed-off-by: Shraddha Agrawal <shraddhaag@ibm.com>