REORG: checks: put the functions in the appropriate files !

REORG: checks: put the functions in the appropriate files !

Checks.c has become a total mess. A number of proxy or server maintenance
and queue management functions were put there probably because they were
used there, but that makes the code untouchable. And that's without saying
that their names does not always relate to what they really do!

So let's do a first pass by moving these ones :
  - set_backend_down()       => backend.c
  - redistribute_pending()   => queue.c:pendconn_redistribute()
  - check_for_pending()      => queue.c:pendconn_grab_from_px()
  - shutdown_sessions        => server.c:srv_shutdown_sessions()
  - shutdown_backup_sessions => server.c:srv_shutdown_backup_sessions()

All of them were moved at once.