If single-value pools have a weight defined, it will be lost by this
optimization. Next time octodns-sync is run, it will show an update for
setting the weight on remote. To overcome this, this commit includes a
change to Record object that ignores the weight in single-value pools.

Use fqdn package to help verify if the record value is really valid.

The original behavior will treat value like `_.` or `.` be a valid
record, which is strange, and the real world may not have those use
cases at all.

The RFC documents are pretty long, as I didn't read them all or enough
to tell should it be valid or not by the spec, so I opened issue #612 to
discuss this case and got a positive response from the main maintainer
to have the change.

Close #628

Closes #626