[optqs] basic fallback heuristic for optimistic quorum store

[ibalajiarun]

Description

A basic implementation of fallback heuristic for optimistic quorum store. Proposal Generator will maintain a list of last proposal statuses (same as NewRoundReason). A exponential window based algorithm to decide whether to go optimistic or not.

Initialize the window at 2.

• For each proposal failure, double the window up to a MAX size
• If there are no failures within the window, the propose optimistic batch
• If there are no failures up to MAX proposals, reset the window to 2.

Note that we don't check whether a round failed due to optimistic quorum store as we don't have that information locally to decide if a proposal failed due to batch missing, unless we collect it globally, so we have to work with local information for now.