Treat a WAL sender process that hasn't started streaming yet as a regular

Treat a WAL sender process that hasn't started streaming yet as a regular
backend, as far as the postmaster shutdown logic is concerned. That means,
fast shutdown will wait for WAL sender processes to exit before signaling
bgwriter to finish. This avoids race conditions between a base backup stopping
or starting, and bgwriter writing the shutdown checkpoint WAL record. We don't
want e.g the end-of-backup WAL record to be written after the shutdown
checkpoint.