Speed up CIDR matching from $wgSquidServersNoPurge

This adds a new generic library class IPSet, which precomputes
a reasonably-efficient data structure from the input list of
addresses and CIDR net/mask specs for fast runtime matching,
and then uses it to check trusted XFF-setters.

See also: 32b4f19f173fc5cff1029eedee63a39a2d72dd3a
Bug: 57021
Change-Id: Ia3b12fb90c3e7e492374a128943b014481cc2730