Add ps display while waiting for wal in read_local_xlog_page_guts

Hi,

pg_create_logical_replication_slot can take longer than usual on a standby
when there is no activity on the primary. We don't have enough information
in the pg_stat_activity or process title to debug why this is taking so
long. Attached a small patch to update the process title while waiting for
the wal in read_local_xlog_page_guts. Any thoughts on introducing a new
wait event too?

For example, in my setup, slot creation took 8 minutes 13 seconds. It only
succeeded after I ran select txid_current() on primary.

postgres=# select pg_create_logical_replication_slot('s1','test_decoding');

pg_create_logical_replication_slot
------------------------------------
(s1,0/C096D10)
(1 row)

Time: 493365.995 ms (08:13.366)

Thanks,
Sirisha