[RFE] Add support to multithread

As per pgbouncer documentation: "PgBouncer is single-threaded and uses one CPU core per instance." [0]

This feature has been added to pgbouncer 1.12, so we should limit it to Focal+.

The solution to set multithreaded pgbouncer is to deploy several processes of it, with slightly different configuration files.

In this, case, set them all with:

so_reuseport = 1 [0]

So these processes will share the same listener port and watch for requests there.

Each pgbouncer should get its own service file (in this case, pgbouncer@<NUM>.service) with a configuration file (/etc/pgbouncer/pgbouncer<NUM>.ini) pointing to a different [0]:

1) PID file: pidfile = /var/run/pgbouncer<NUM>.pid
2) log file: logfile = /var/log/postgresql/bouncer<NUM>.log
3) socket: unix_socket_dir = /var/run/postgresql/bouncer<NUM>_sock

We should also add a config option to define the number of worker processes:

config:
  worker_processes:
    type: int
    default: 0
    description: |
      Set the number of concurrent pgbouncer processes running in the host.
      If set to 0, pgbouncer count == # of cores.
      If set to 1, one single pgbouncer will be set with "so_reuseport=0" option
      If set to N>1, then N pgbouncers will be deployed

[0] https://www.pgbouncer.org/config.html