Allow dismissal of proposals?

[fabriziovigevani]

Created this issue to keep in the backlog just in case.

Based on @lkngtn #27 (comment)

In terms of users being able to "exit" using the acl oracle. It should be possible to exit after 1/2 the buffer duration has passed even if the vote has not been executed. The 1/2 period is intended to ensure users are both locked in for votes they voted yes on, but still have an opportunity to exit before the next vote that they didn't vote yes on gets executed.

The idea here is that, the 1/2 buffer duration give other members an opportunity to execute the vote before anyone who voted yes on the proposal has the opportunity to exit.

If we are extra paranoid, it may be possible for an attacker to get around this by passing a proposal that will fail to execute until after 1/2 the buffer duration, but will be able to execute after the duration. In this case for safety it may make sense to allow a vote to be dismissed if some condition is met. We could say that if the current token supply the vote is based on is x% of the supply at the block the proposal started the vote can be dismissed rather than executed.