call dns_resolver_createfetch() asynchronously in zone_refreshkeys()

because dns_resolver_createfetch() locks the view, it was necessary to unlock the zone in zone_refreshkeys() before calling it in order to maintain the lock order, and relock afterward. this permitted a race with dns_zone_synckeyzone().

this commit moves the call to dns_resolver_createfetch() into a separate function which is called asynchronously after the zone has been unlocked.

Closes #3617 (closed)