[server] implement dynamic database pooler
Opening too much database connection has a cost at startup and also PostgreSQL
as a maximum number of connection (100 by default).
This get worse when starting multiple wsgi processes, since each process has
its own database pool.
Instead of opening `connections-pool-size` connections to the database at
startup, just open one and open more only when needed.
.. _resources:
Locate resources
----------------
.. automethod:: cubicweb.web.webconfig.WebConfiguration.locate_resource
.. automethod:: cubicweb.web.webconfig.WebConfiguration.locate_doc_file
.. automethod:: cubicweb.web.webconfig.WebConfiguration.locate_all_files
Static files handling
---------------------
.. autoattribute:: cubicweb.web.webconfig.WebConfiguration.static_directory
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_exists
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_open
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_add
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_del