Included changes:

- log warnings properly
- issue warning when FQDN retrieval fails for system.identity and system.nsid
- cache hostname lookup

refs #306

Original commits:

9de0e69e  Merge branch 'conf-hostname-error' into 'master'
7bcf3ff3  log: cache hostname lookup for nsid and identity
9ab4ea4f  log: warn on error retrieving auto value for NSID and identity
3ad605d6  log: update warnings to use cf_warning
87b7676c  conf: added cf_warning API

expire zone when XFR is failing

fixes #304

See merge request !297

Forced refresh fix

Fixes #303

See merge request !298

- Serials are checked during SOA response processing. Incoming
  AXFR should be accepted even if the serial is older. (In order
  to allow forced refersh.)

fixes: persistent timers

See merge request !294