Check if database is really alive

[amineau]

Adding a check for postgresql to avoid the loss of a request.

The connection.cursor() opens a connection for CONN_MAX_AGE time if it is not already open.
Otherwise, the old cursor is used and no verification is done.

This is inspired by the is_usable() method of django.
https://github.com/django/django/blob/76c0b32f826469320c59709d31e2f2126dd7c505/django/db/backends/postgresql/base.py#L298