Avoid preemptive DB replication waits for farm cross-wiki redirects

This previously only worked if $wgLocalVirtualHosts was set, which
was too specific to check and not used by WMF. Use the more generic
WikiMap class.

Two methods have been added there to do the work of enumerating
canonical wiki farm URLs and checking them against a given URL.

Bug: T172357
Change-Id: Id2415bab5d7f5a08b9f536858c32d329138384a2