Prevent false challenges from short-circuiting the challenge period

[ameensol]

As it stands, it is possible for a channel participant to quickly, independently:

1. Propose a checkpoint
2. Submit a valid challenge, but one with a lower impressions count than the proposed state
3. Accept their own challenge, finalizing the initial proposed state

This would allow them to short-circuit the challenge period during checkpointing, and get away with finalizing a checkpoint with an old state.

[ameensol]

Possible fixes:

1. don't allow the checkpoint proposer to challenge their own proposed checkpoint
2. don't allow the challenge accepter to accept their own challenge
3. don't immediately finalize the checkpoint if the challenge is successfully accepted, instead extend the challenge period
   a. this permits infinite challenge loops
4. split the challenge period into two distinct phases (challenging, accepting challenges)
   a. e.g. can only submit a challenge within phase 1, can only accept challenge within phase 2

I'm leaning towards 1 + 2 because they solve the problem with the least work.