Fix proof generation for subsequential polls

[corydickson]

Instead of allowing concurrent polls where users must signup for each poll, this PR aims to allow on chain proof verification for a multiple poll instances with the same set of users.

Testing run cli/testMultiplePoll.sh

[corydickson]

Without changing how the sbCommitments are updated, this command is still necessary even though the StateAq subtrees are merged.

[weijiekoh]

Hi! Does this include #311 or should that be merged separately?

[weijiekoh]

PollFactory compiles to 25Kb which exceeds the mainnet limit

[weijiekoh]

Suggestions to reduce the size of Poll.sol:

• Comment out batchEnqueueMessage which isn't used - size is now 24.57 Kb
• Have a separate ResultVerifier contract that implements verifySpentVoiceCredits, verifyPerVOSpentVoiceCredits, verifyTallyResult, and computeMerkleRootFromPath, instead of implementing these functions in Poll.sol.

We should reduce the size of Poll.sol before merging this PR

[weijiekoh]

Looks good and the test script works on my end too. We just need to reduce the size of Poll.sol - see the previous comment on how we can do this.

Edit: let's do the size reduction in a different PR.