vinyl: log new index before WAL write on DDL

Currently, we write new indexes to vylog only after successful WAL
write (see vinyl_index_commit_create). This is incompatible with space
ALTER - the problem is during ALTER vinyl may need to create new run
files, which we need to track in order not to leave garbage if ALTER
fails or tarantool exits before ALTER is complete.

So this patch splits index creation in two stages, prepare and commit.
The 'commit' stage is represented by existing VY_LOG_CREATE_LSM record,
which is written from index_vtab::commit_create callback, just like
before. For the 'prepare' stage we introduce a new record type,
VY_LOG_REPARE_LSM, written from index_vtab::add_primary_key and
index_vtab::build_index callbacks, i.e. before WAL write. For now, we
don't write anything to prepared, but uncommitted indexes (this will be
done later), but we do add prepared indexes to the scheduler so that
they can be dumped and compacted. If ALTER fails, we drop prepared
indexes in index_vtab::abort_create callback. Prepared but uncommitted
indexes are ignored by backup and replication and cleaned up from vylog
on restart.

Note, we have to rework vinyl/errinj_vylog test in this patch, because
index creation (and hence space truncation) commands now fail on vylog
error, i.e. a situation when the same index is dropped and recreated
multiple times in xlog without having corresponding records in vylog is
now impossible.

Also, space truncation is not linearizable for vinyl anymore as it may
yield before WAL write, while trying to prepare an index in vylog. This
is OK - we never promised it is. Remove the corresponding test case.

Needed for #1653