Excessive number of save() operations for Zone objects

[peteeckel]

Versions
NetBox DNS Version: 0.22.4 and before

Describe the bug
While working on #89 it became obvious that saving modified a Zone object can result in an excessive number of additional save() operations for the object itself and other zones related to it, e.g. reverse zones and RFC2317 zones.

Essentially every save() to a Record objects causes a subsequent save() to its associated zone because the SOA serial number of the zone needs to be updated. Since a save() operation on a zone object can result in the creation of multiple records, or in some cases many records (i.e. when a reverse zone is created, deleted, or moved), this can be a very substantial performance issue at times.