MEDIUM: proxy: remove state PR_STPAUSED

MEDIUM: proxy: remove state PR_STPAUSED

This state was used to mention that a proxy was in PAUSED state, as opposed
to the READY state. This was causing some trouble because if a listener
failed to resume (e.g. because its port was temporarily in use during the
resume), it was not possible to retry the operation later. Now by checking
the number of READY or PAUSED listeners instead, we can accurately know if
something went bad and try to fix it again later. The case of the temporary
port conflict during resume now works well:

  $ socat readline /tmp/sock1
  prompt
  > disable frontend testme3

  > disable frontend testme3
  All sockets are already disabled.

  > enable frontend testme3
  Failed to resume frontend, check logs for precise cause (port conflict?).

  > enable frontend testme3

  > enable frontend testme3
  All sockets are already enabled.