update to primary/secondary terminology internally

"primary" and "secondary" have been added as synonyms and the documentation and test configurations updated to use it. this MR changes the terminology used internally as well; for example, dns_zone_master and CFG_ZONE_MASTER are now dns_zone_primary and CFG_ZONE_PRIMARY respectively.

the statistics channel has been updated to emit zone types using the new terminology as well. this change may affect scripts that parse statistics output, and so should probably not be backported even if the rest of the branch is.

Closes #1944 (closed)