alter: commit old index drop and new index create on index rebuild

To rebuild an index when its key def changes, we effectively drop it and
create a new index instead. Skipping Index::commitDrop and commitCreate
stages at this point deprives Vinyl of an opportunity to log the change
in the metadata log and replace the index in the scheduler, which leads
to a crash. This patch adds the commit stage to RebuildIndex which calls
the above-mentioned commitDrop and commitCreate for the old and the new
indexes, respectively.

There is a nuance here. Memtx piggybacks Index::commitDrop to drop space
tuples when the primary index is dropped. This is actually wrong for
tuples belong to a space, not to an index. Besides, it prevents us from
just calling Index::commitDrop() from RebuildIndex::commit() as is,
because RebuildIndex does not modify space data, it just moves space
tuples to a new index. To circumvent this, let us remove commitDrop()
method from MemtxIndex and drop space tuples directly from MemtxSpace's
commitTruncateSpace() and commitAlterSpace().