Explore ordering algorithm

[fubuloubu]

Sketch of an idea:

• have each proposal contain a [parallel] set of proposal hashes that must be executed first (NOTE: "parallel" here means "not a part of the proposal, but committed in parallel to it)
• Set can be changed at any time, but it resets the approvals
• When executing, for each hash in the set, iterate backwards through the queue of previously-executed proposals to search for a match
• If proposal has not been found in queue, revert

This should work even for large size queues because typically, the setting is for usually 10 or less items to clear out the queue. If the number gets too large, you may experience gas exhaustion attacks, but the solution to that is to clear the pending transactions and reset the set of IDs so that it is much smaller and more recent. This also holds for when there is mutual reference between two proposals