vinyl: do not reallocate tuple formats on alter

We create new formats for all indexes of the new space in
vinyl_space_commit_alter() while we don't actually need to
do this, because the new formats have already been created
by vy_lsm_new() - all we need to do is reuse them somehow.

This patch does the trick: it implements the swap_index()
space virtual method for vinyl so that it swaps tuple formats
between the old and new spaces.