Use gmtime instead of localtime when generating SOA serials

closes #1339

Conflicts:
	pdns/dnsseckeeper.hh

This is just a rebased version of https://github.com/abh/pdns/commit/e5f92e2a77b1d8a459b50ea7ec6098687cf594b8 -- the older commit is what I've been using successfully in production; this version I haven't had a chance to try.