Open up `transfer` for partial checkouts

[NiloCK]

All public api methods for moving funds out of a channel currently _requireChannelFinalized. This prevents partial checkouts.

Partial checkouts are an important piece to permit lowest-cost happy path nitro, where intermediaries selectively transfer from their spent client ledger channels directly into provider ledger channels in order to top up capacity as it dwindles. (See #1342)

Under this mechanism, service providers will want to partially checkout their running channels in order to 'take profits'.