Use WAIT FOR LSN in PostgreSQL::Test::Cluster::wait_for_catchup()

Use WAIT FOR LSN in PostgreSQL::Test::Cluster::wait_for_catchup()

When the standby is passed as a PostgreSQL::Test::Cluster instance,
use the WAIT FOR LSN command on the standby server to implement
wait_for_catchup() for replay, write, and flush modes.  This is more
efficient than polling pg_stat_replication on the upstream, as the
WAIT FOR LSN command uses a latch-based wakeup mechanism.

The optimization applies when:
- The standby is passed as a Cluster object (not just a name string)
- The mode is 'replay', 'write', or 'flush' (not 'sent')
- The standby is in recovery

For 'sent' mode, when the standby is passed as a string (e.g., a
subscription name for logical replication), or when the standby has
been promoted, the function falls back to the original polling-based
approach using pg_stat_replication on the upstream.

Discussion: https://postgr.es/m/CABPTF7UiArgW-sXj9CNwRzUhYOQrevLzkYcgBydmX5oDes1sjg%40mail.gmail.com
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@kurilemu.de>