[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.

3.28
====

Changes
-------

- the class cubicweb.view.EntityAdapter was moved to cubicweb.entity.EntityAdapter
  a deprecation warning is in place, but please update your source code accordingly.

- The database pooler is now dynamic. New connections are opened when needed and closed after a configurable period of time. This can be configured through `connections-pooler-max-size` (default 0, unlimited), `connections-pooler-min-size` (default 0), and `connections-pooler-idle-timeout` (default 600 seconds). The old configuration `connections-pooler-size` has been dropped.