Michał Kępień authored Jan 16, 2019 and Evan Hunt committed Jan 16, 2019

Log a message when a mirror zone is successfully loaded from disk and
subsequently verified.

This could have been implemented in a simpler manner, e.g. by modifying
an earlier code branch inside zone_postload() which checks whether the
zone already has a database attached and calls attachdb() if it does
not, but that would cause the resulting logs to indicate that a mirror
zone comes into effect before the "loaded serial ..." message is logged,
which would be confusing.

Tweak some existing sed commands used in the "mirror" system test to
ensure that separate test cases comprising it do not break each other.