Flesh out the test set for SRV-only mode and fix bugs thus discovered:
 Prevent scheduled TTL refresh code from marking DDDS search as recently used
 Fix synthetic SRV owner name accumulating multiple prepends
 Fix scheduled TTL refresh code to refresh SRV belonging to synthetic NAPTR
 Fix scheduled TTL refresh for failed/negative A/AAAA records
 Emit debug on scheduled TTL refresh of A/AAAA records, not just opportunistic.
 Fix off by one error to allow MAX_SRVS SRV RRs, not MAX_SRVS - 1
 Use WDEBUG for SRV RRs exceeded so it is colorized