When splitting huge unmap requests, do it on sector boundary.
Introduce new serialization type CTL_SERIDX_UNMAP.

Unfortunately we can't check range collisions for UNMAP commands alike
to writes, because they include multiple ranges, which are also passed
in data block, not in CDB.  As result, UNMAP commands have to be treated
as colliding with any other command accessing the media.

From the other side all UNMAPs are equal (we don't support ANCHOR flag),
so we can execute several UNMAPs same time.