alter: propagate truncate_count to new_space

On space alter (index create/drop) new_space->truncate_count is left
unset, which breaks vinyl recovery logic: in vinyl we assume that if
index->truncate_count (restored from vylog) > space->truncate_count,
the space will be truncated and so there's no need in applying a WAL
row (see vy_is_committed_one()). So if an index is dropped or created
after truncate, vinyl won't apply statements for it on recovery.

Closes #2615