Multi ledger

[matthiasgeihs]

Enables multi-ledger channels.

The following changes are applied:

• Implement special Asset type that contains LedgerID.
• Implement multi funder (adjudicator) that contain a set of funders (adjudicators) which are called depending on which assets a channel has.
• Revise watcher: The watcher ensures that in case of a multi-ledger channel, all watched ledgers are kept synchronized.

[RmbRT]

As I addressed above, I think the general architecture and protocols need to be adapted a little more to actually support multi-ledger channels. Architecture-wise, it looks good to me so far. As I also suggested in a meeting, it would maybe be expedient if we migrated the core to multi-ledger but added a legacy single-ledger adapter for older backends.
Also, even when only rolling out non-force-move channels, the on-chain contracts would need an option in the channel params that allows the user to disable force-moving.

[matthiasgeihs]

I have addressed all comments. Significant changes:

• Restrict usage of multi-ledger to non-app channels.
• Moved package multi from backend to channel.

[matthiasgeihs]

Addressed all comments. Here is the diff: https://github.com/hyperledger-labs/go-perun/compare/5c0ec670f3b19ab88de2a54925f8fd6575374c42..aeae7717a81d16cd268add2d769f6176814e49c6

Please check and approve if you are happy. @cryptphil @RmbRT