alter: rewrite space truncation using alter infrastructure

Truncation of a space is equivalent to recreation of all space indexes
with the same definition. The reason why we use a special system space
to trigger space truncation (_truncate) is that we don't have
transactional DDL while space truncation has to be done atomically.
However, apart from the new system space, implementation of truncation
entailed a new vylog record (VY_LOG_TRUNCATE_INDEX) and quite a few
lines of code to handle it. So why couldn't we just invoke ALTER that
would recreate all indexes?

To answer this question, one needs to recall that back then vinyl used
LSN to identify indexes in vylog. As a result, we couldn't recreate more
than one index in one operation - if we did that, they would all have
the same LSN and hence wouldn't be distinguishable in vylog. So we had
to introduce a special vylog operation (VY_LOG_TRUNCATE_INDEX) that
bump the truncation counter of an index instead of just dropping and
recreating it. We also had to introduce a pair of new virtual space
methods, prepare_truncate and commit_truncate so that we could write
this new command to vylog in vinyl. Putting it all together, it becomes
obvious why we couldn't reuse ALTER code for space truncation.

Fortunately, things have changed since then. Now, vylog identifies
indexes by space_id/index_id. That means that now we can simplify
space truncation implementation a great deal by

 - reusing alter_space_do() for space truncation,
 - dropping space_vtab::prepare_truncate and commit_truncate,
 - removing truncate_count from space, index, and vylog.