BUG/MEDIUM: proto_udp/threads: recv() and send() must not be exclusive.

BUG/MEDIUM: proto_udp/threads: recv() and send() must not be exclusive.

This is a complement to previous fix for bug #399. The exclusion between
the recv() and send() calls prevents send handlers from being called if
rx readiness is reported. The DNS code can trigger this situations with
threads where the fd_recv_ready() flag disappears between the test in
dgram_fd_handler() and the second test in dns_resolve_recv() while a
thread calls fd_cant_recv(), and this situation can sustain itself for
a while. With 8 threads and an error in the socket queue, placing a
printf on the return statement in dns_resolve_recv() scrolls very fast.

Simply removing the "else" in dgram_fd_handler() addresses the issue.

This fix must be backported as far as 1.6.

(cherry picked from commit d7f76a0a50f4ac6b32d2317c675b3752133ef6d2)
Signed-off-by: Willy Tarreau <w@1wt.eu>