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

support them

That's all Dyn supports and it's cleaner to match it than to scale dyn since
we'd lose precision we can't get back during populate.

Route53 allows to monitor latency information on the dashboard
and using CloudWatch. While that is a nice to have function,
it is not necessary for a DNS failover scenario and increases
Route 53 costs.
To maintain backward compatibility, the default for this option
when ommited is true.