vinyl: do not delete shadow range files when collecting garbage

We collect garbage by deleting all files in the index directory that
aren't referenced by any range (see vy_index_gc). We assume a file to be
referenced, if there's a range in the index->tree whose id matches the
file name. Since shadow ranges (used for compaction) are expelled from
the tree, their files might be removed as well, which would result in
data loss if compaction was aborted. Fix that by adding shadow ranges to
the the temporary map used for filtering out active ranges.