Introduce min ballot duration in VotingToManageEmissionFunds contract

[maxaleks]

#190

[varasev]

Please also review require statements related to checking the _startTime and _endTime parameters in the VotingToManageEmissionFunds.createBallot function. Since now we use the onlyValidTime modifier inside the VotingTo._createBallot internal function, those require statements in VotingToManageEmissionFunds.createBallot look redundant.

[varasev]

Let's also add canBeFinalizedNow into this condition:

poa-network-consensus-contracts/contracts/VotingToManageEmissionFunds.sol

Line 223 in f7a3745

if (getTotalVoters(_id) >= validatorsLength && !_withinCancelingThreshold(_id)) {

and add an additional unit tests to test/voting_to_manage_emission_funds_*.js for the VotingToManageEmissionFunds.finalize, vote, and init functions to cover the added code by tests.

[varasev]

I've checked the changes with poa-test-setup and found no problems, so I'm approving this PR.