Support for server pools in minidns

notify_zone_changed and poll_for_serial_number
are now implemented.
Currently mdns has some temporary code to allow
both pool manager and central to call into it.  This
is to ensure that nothing breaks while the pool
manager is still under development.

Partially-implements: blueprint server-pools-minidns-support
Change-Id: I1977b4f910e986e876c9a4e07827d4f84a6cfd78