refresh: Removed SOA check from AXFR processing

- Serials are checked during SOA response processing. Incoming
  AXFR should be accepted even if the serial is older. (In order
  to allow forced refersh.)
tests-extra: refresh -f test
test-extra: fix Knot.ctl()
tests-extra: Improved/fixed test for forced refresh
refresh: Improved log message in case of older IXFR
refresh: Warning instead of Info
Merge branch 'refresh' into '1.6'

Forced refresh fix

Fixes #303

See merge request !298