Add PostgreSQL::Test::Cluster::advance_wal

Add PostgreSQL::Test::Cluster::advance_wal

This is a function that makes a node jump by N WAL segments, which is
something a couple of tests have been relying on for some cases related
to streaming, replication slot limits and logical decoding on standbys.
Hence, this centralizes the logic, while making it cheaper by relying on
pg_logical_emit_message() to emit WAL records before switching to a new
segment.

Author: Bharath Rupireddy
Reviewed-by: Kyotaro Horiguchi, Euler Taveira
Discussion: https://postgr.es/m/CALj2ACU3R8QFCvDewHCMKjgb2w_-CMCyd6DAK=Jb-af14da5eg@mail.gmail.com