BFT RPC interface

[achamayou]

For users to get BFT guarantees on their transactions, they must obtain a valid receipt. Just polling tx for status is insufficient.

This isn't communicated particularly well in documentation or in interface at the moment, so this discussion is calling for ideas and suggestions.

Additionally, there's the matter of read transactions. While BFT can execute some through the consensus and produce a specific receipt, it isn't the case for others such as historical queries. Rather than hardcode this with ExecuteLocally, @ashamis suggested we might take a custom header such as Read-Receipt: false to tell CCF whether to execute through the consensus or not. Endpoints that do not support it (eg. historical queries) would then return an error if this isn't set correctly.