Reliable Store Protocol: Enforcing view consistency across store nodes

[staheri14]

Currently, store nodes work independently without synchronizing the state of their persisted messages. As such, there might be situations that they may have different views of the historical messages. This view inconsistency also means that light nodes cannot rely on the completeness of the history provided by a single store node.

This issue is to provide a mechanism to synchronize the state of store nodes and enable them to exchange their views and converge to a consistent and complete state. This will add reliability to the overall store protocol service. Moreover, any single full store node becomes a reliable source of message history.

The initial solution idea #73