FM-196: Implement ProposalInterpreter::process for BottomUp

[aakoshh]

Closes consensus-shipyard/ipc#308
Depends on #201 and #185

Implements ProposalInterpreter::process method for ChainMessageInterpreter by looking up proposed checkpoints for execution in the ResolvePool and checking that the item is present and resolved, otherwise we don't vote for the proposal.

[adlrocha]

LGTM. One quick question (and I think I saw a message for this). The checkpoint pool will be garbage collected after a checkpoint execution, right? At this point we know it won't have to be re-proposed anymore.

[aakoshh]

The checkpoint pool will be garbage collected after a checkpoint execution, right? At this point we know it won't have to be re-proposed anymore.

That's right, when the checkpoint is executed it has to be removed from the resolver pool. Thanks for mentioning this because I have not included this in any of the issue description, just commented on it in the code. I created consensus-shipyard/ipc#227 to track it now.