Incremental indexing fixes from Eric W. Biederman.
These prevents the highest message number in msgmap from
being reassigned after deletes in rare cases and ensures
messages are deleted from msgmap in v2.
* eb/index-incremental:
V2Writeable.pm: In unindex_oid delete the message from msgmap
V2Writeable.pm: Ensure that a found message number is in the msgmap
SearchIdx,V2Writeable: Update num_highwater on optimized deletes
t/v[12]reindex.t: Verify the num highwater is as expected
t/v[12]reindex.t Verify num_highwater
Msgmap.pm: Track the largest value of num ever assigned
SearchIdx.pm: Always assign numbers backwards during incremental indexing
t/v[12]reindex.t: Test incremental indexing works
t/v[12]reindex.t: Test that the resulting msgmap is as expected
t/v[12]reindex.t: Place expected second in Xapian tests
t/v2reindex.t: Isolate the test cases more
t/v1reindex.t: Isolate the test cases
Import.pm: Don't assume {in} and {out} always exist