[server] Make connection pooler configurable and set better default values Drop the configuration connections-pool-size and add new configurations options: * connections-pool-min-size. Set to 0 by default so we open connections only when needed. This avoid opening min-size*processes connections at startup, which is, it think, a good default. * connections-pool-max-size. Set to 0 (unlimited) by default, so we move the bottleneck to postgresql. * connections-idle-timeout. Set to 10 minutes. I don't have arguments about this except that this is the default in pgbouncer.

[server] Enhance connections-pooler-enabled documentation

[server] move connection pooler initialization logic to get_cnxset() This avoid complex logic in Repository initialization.