dnsdist: Add an option to use several source ports toward a backend

dnsdist: Add an option to use several source ports toward a backend

This is very useful if the backend is distributing queries based
only on (source IP, source port, destination IP, destination port),
which is for example the case of PowerDNS Recursor with several
threads, reuseport set and pdns-distribute-queries not set.

(cherry picked from commit 150105a20eaebc8e0041b1a41b81fd90d7dbaba3)
(cherry picked from commit e998def2a9007aab462c90ff5badc2eab7253ee8)
(cherry picked from commit 38069e7ea3ad11b1cc055469cf3378531f2e7239)
(cherry picked from commit cd73ceebff1b805de167ad23198def0c8a59d786)
(cherry picked from commit 5bdbb83d3b2a6aabb9a4f36f7b0a2ee0acca2f15)