Fairness across channels & connections

[cwgoes]

Split off from #126.

Possible sorts of fairness:

• Fairness across all connections on a chain
• Fairness across set of connections between two chains
• Fairness across set of channels on a connection

Priority of given message determined by sender, costs may vary according to priority.

Note that we can at most restrict validator ordering choices, and neither connections nor channels are by default scarce, so scarcity would need to be imposed (with cost, or escrowed deposits required to open a connection / channel).