MEDIUM: session: close new idle conns if server in maintenance

MEDIUM: session: close new idle conns if server in maintenance

Previous patch ensures that a backend connection going into idle state
is rejected and freed if its target server is in maintenance.

This patch introduces a similar change for connections attached in the
session. session_check_idle_conn() now returns an errorl if connection
target server is in maintenance, similarly to session max idle conns
limit reached. This is sufficient to instruct muxes to delete the
connection immediately.