Memdb skips cleaning on disk snapshots on drop index

Memdb skips cleaning on disk snapshots on drop index.

One possibility is slice refcount doesn't go down to 0 due to pending scans.

It can be seen for index 6295153656074343086, on drop index, slice is only softdeleted. This indicates there are other readers referencing the slice.

2016-03-09T14:19:11.516-08:00 [Info] MemDBSlice::Close Closing Slice Id 6295153656074343086, IndexInstId 6295153656074343086,
IndexDefnId 0
2016-03-09T14:19:11.516-08:00 [Info] MemDBSlice::Destroy Softdeleted Slice Id 0, IndexInstId 6295153656074343086, IndexDefnId 6295153656074343086

CBQ is running in background and from the stats just before the drop, it can be seen that there are pending scans

"default:i1:num_requests":122961
"default:i1:num_completed_requests":121931

Indexer log is attached.