Keyvalue updates read and write to storage

[tabcat]

Now that the replica/graph state is kept on disk the state of the database's replica is available immediately. The same needs to be done for the keyvalue stores index. When completed the index should be able to be queried immediately, as opposed to loading and processing the entries again.

Behavior:

• read the store index on start without loading entries into memory
• compute new index and write root hash to storage (not doing perf improvements yet)