Propagate TTLs from new messages into the cache.

[blakej11]

I've tried using a few Golang libraries for mDNS in the past couple days. I found https://github.com/grandcat/zeroconf and then this one, and I noticed that this one actually does things with the TTLs returned with mDNS entries. That appears to be because this fork has this commit, which the parent repo doesn't have.

When experimenting with it, I discovered that this updated TTL support doesn't quite work. Specifically, (*client).mainloop accumulates new responses into the entries map, and once it has sent information about a new service to the subscriber, it stores a copy of it in the sentEntries map. It has code to delete entries from the sentEntries map once they expire. But if it receives updated information for an entry, it doesn't propagate the new Expiry time to the entry in sentEntries, so that entry will be missing between the expiry of the original message and when it receives a new message for it.

This PR has a simple fix for this - propagate the updated Expiry time from any new entries that it receives.

I've seen comments in other PRs saying "please submit this to the parent, we don't want to deal with the maintenance burden". As far as I can tell, this issue is specific to your fork, so I think this is the right place to send it. Also, if you don't want it, no worries.

[blakej11]

@marten-seemann tagging you for consideration of this PR - I think this is a bug in your fork of the zeroconf library. If you don't want to take it, that's totally fine, but I thought I'd offer.