Cope with duplicate name->IP entries

The zone DB no longer considers duplciates of (IP, name); it will
reject duplicates of (IP, name, ident), but this ultimately doesn't
matter since those are all the fields it records anyway.

The HTTP interface treats these duplicates as trivial success (i.e.,
it acts as if it's idempotent, which it is).

This changes the behaviour for removals: if (IP, name) is recorded for
two different containers, removing one will leave the other extant.
Merge pull request #257 from squaremo/issue249_duplicate_dns

LGTM.  Closes #249