2nd level index

[arkpar]

Introduce a second level hash index. This will be used to store smart contract tries, child tries, etc. The main requirement here is that the sub-index can be dropped in amortized constant time.

API requirements:
The hash column can be configured to use a second level index. All insert/get commands for such column accept two keys: K1 and K2 where K1 identifies the index, and K2 is the value key in that index. Additionally, there's a drop command that accepts a single key and deletes all values in the index identified by the key.

Implementation ideas are welcome. As far as I see it, the structure could be made similar to existing index with all values in a single index. Deletion could be done by background process, similar to re-indexing. When an index is dropped, it is added to the deletion queue. A background process checks all index entries and deletes any values that are in the indexes being dropped.