RetrievalProviderNode, StorageProviderNode, RetrievalClientNode, and StorageClientNode should provide a TipSet identifier when querying blockchain

[laser]

Why does this ticket exist?

When a provider or client needs some information from the chain, they need to provide the a tipset identity such that a sequence of queries which are intended to be against a single chain state are guaranteed to be against the same chain state.

Note: A tipset height does not serve as identity due to re-orgs (tipset X at height 10 may be reorged out of existence, replaced with tipset Y).

Acceptance Criteria

• each *Node interface must expose a method used to get the current chain head's tipset identity
• each Get* method (used to query chain state) must accept a tipset identity to which the query is scoped