Add secondary index support to SnapshotMap.

[ethanfrey]

Extend the SnapshotMap with a secondary index. The historical queries (snapshots) are only for the primary key. The secondary index is not snapshotted. Maybe this needs a new type (IndexedSnapshotMap) that wraps a snapshot map?

This would be useful for eg. adding a secondary index based on weight to any of the group contracts that use a snapshot map. But in general to allow composibiity between these two advanced functions (snapshots and indexes) allows much more power to storage-plus