MINOR: queue: reduce the locked area in pendconn_add()

MINOR: queue: reduce the locked area in pendconn_add()

Similarly to previous changes, we know if we're dealing with a server
or proxy lock so let's directly lock at the finest possible places
there. It's worth noting that a part of the operation consisting in
an increment and update of a max could be done outside of the lock
using atomic ops and a CAS.