Feature: Include poll start time as poll parameter

[lordshashank]

Prerequisites

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Motivation

While configuring polls people prefer to start poll in future rather than instantly, having this feature would enable people to have similar feature of delayed polls as in regular off-chain polls, elections, etc purposes we aim to serve with MACI.

Details

Currently MACI poll have a parameter of duration which allows us to configure duration of poll. However it leads to problem that poll has to start at the time of creation i.e. people can cast vote, etc from that until duration. We would need to configure another parameter, something like startEpoch that takes in epoch number to start poll and block votes, messages, etc until that epoch gets passed. This parameter would be passed at the time of poll creation by user.