Funding Round contract: Add method for batch submission of messages

[xuhcc]

To improve contributor's experience, we need a method for batch submission of messages. I think it should be implemented in FundingRound contract because the size of MACI contract is already approaching limits.

function submitMessageBatch(Message[] memory _messages, PubKey[] memory _encPubKeys)

It seems that we also need to ensure that the first message is a key-change command.

https://github.com/appliedzkp/maci/blob/master/specs/01_introduction.md#high-level-process

A later version of MACI will have to mitigate a vote-buying attack where a user may be bribed immediately when the voting period starts, such that their very first message is influenced by a briber. One solution to this is to have the contract to mandate that the first message that each participant sends is to change their key (see below).