Switched to 'server' category from 'zone' category,
because the zone name from the configuration is not
available during module initialization.

- log_msg() logs into 'server' category
- log_msg_zone() and log_msg_zone_str() log into 'zone' category

libknot dependencies cleanup

Divided files from `common` to `common` and `common-knot`, some moved to `libknot`.

Only those in `common` are used in `libknot` and only in .c files. `libknot` headers are no longer dependent on any headers outside `libknot`.

See issue #199

See merge request !245

Axfr denied log

See merge request !248

fixes #272

This reverts commit 5a9d102c.

Restart tests

Review changes to the testing framework, please.

See merge request !247