[contracts] Investigate batched withdrawals

[rhlsthrm]

This will greatly reduce router costs.

This is likely not possible with our current architecture.

Idea: Use ChannelFactory.sol to perform batch withdrawals.

• Pass an array of withdrawal commitment data.
• For each withdrawal commitment, check if the channel needs deployment, if so, deploy (possibly with deposit).
• Submit withdrawal.

Need to check about the security ramifications of this.

[ArjunBhuptani]

For now, let's table this and focus on https://github.com/connext/vector/issues/358 as it involves no contract changes