[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.
### Requirements for tests in various cubicweb/**/test directories. ###
## shared by several test folders
docutils
webtest
## cubicweb/test
Pygments
pycryptodomex
#fyzz XXX pip install fails
## cubicweb/devtools/test
flake8
## cubicweb/hooks/test
psycopg2-binary
## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth
repoze.lru
## cubicweb/sobject/test