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.