[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.
etc/init.d
etc/cubicweb.d
usr/bin
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances