bind-add-zone: add bind zones using pdns_control

Add pdns_control for loading new zones without reloading named.conf.
Zone is added but not loaded, you should do it manually, by querying
something. Beware of cache, it is better to purge it before.
bind-add-zone: Add stress-test for bind-add-zone

Uses dnsperf. See README for details.
bind-add-zone: fix for tests context
bind-add-zone: verbose error message
bind-add-zone: rename test, fix timeout
bind-add-zone: add docs