[store] Introduce Chain store adapter

[shreyan-gupta]

For now this PR only moves some of the functionality from ChainStore to ChainStoreAdapter, the simpler functions.

We also do away with the cache in ChainStore.

In future PRs, will work towards getting a better suited interface with helper functions which are too complex to be a part of ChainStoreAdapter.

Also big issue is for the ChainStoreUpdateAdapter seeing a consistent view of all the updates. Will need to see how to make that happen!