Add a dynamic SOA record (at least the serial and minttl). This create
another interface that should be implemented by the backends.
For now default to returning epoch in as a uint32 (no change from
before).
Lower the minTTL returned to 30s (from 60s)
* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat