tests-extra: add Knot.ctl() to allow arbitrary control operations
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
events: add zone_events_is_scheduled() API
expire zone if SOA is accepted, but AXFR is not
schedule retry after failed AXFR
review: mem_trim() after XFR when old zone is discarded
tests-extra: zone expiration by broken AXFR
Merge branch 'refresh' into '1.6'

Forced refresh fix

Fixes #303

See merge request !298
Merge branch 'fix-expire-noauth-axfr' into '1.6'

expire zone when XFR is failing

fixes #304

See merge request !297
knotc: fix help message formatting
bump version to 1.6.0
Merge branch '1.6'

* 1.6:
  bump version to 1.6.0
  knotc: fix help message formatting
  tests-extra: zone expiration by broken AXFR
  review: mem_trim() after XFR when old zone is discarded
  schedule retry after failed AXFR
  expire zone if SOA is accepted, but AXFR is not
  events: add zone_events_is_scheduled() API
  refresh: Warning instead of Info
  refresh: Improved log message in case of older IXFR
  tests-extra: Improved/fixed test for forced refresh
  test-extra: fix Knot.ctl()
  tests-extra: refresh -f test
  refresh: Removed SOA check from AXFR processing
  tests-extra: add Knot.ctl() to allow arbitrary control operations