[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.
Document: cubicweb-doc
Title: CubicWeb Documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developers
Section: Programming
Format: HTML
Index: /usr/share/doc/python3-cubicweb-documentation/html/index.html
Files: /usr/share/doc/python3-cubicweb-documentation/html/*