[DNS recovery]: only send if sendQueue is small

First patch to dns-recovery branch.

This gaurds against a known unbounded queue in node core.

cc @Matt-Esch @mranney @jcorbin
and when it's not we just silently drop data? it'd be nice to keep a count of messages dropped due to this, which then gets cleared and optionally reported somehow after we fall back under the high water mark... but that could totally be a subsequent PR