Enable other behaviours of `push/pull_distributed`

[barneydobson]

The preference system is not particularly predictable and mainly suitable when no further information is available.

In particular there are two other methods to distribute that would be useful:

• By percentages, i.e., always try to push/pull in direct proportion. In this case the preferences could have a meaning of percentage.
• By absolute, i.e., try to push/pull everything to the highest preference first, any leftover goes to next highest, etc.

I think this will need a refactor somewhere, I think we could get away with only refactoring get_connected. Though it would still need someone to go through and identify places where these new behaviours would be preferrable to the default.