Load management send slow down when close to limit

Send slow-down messages when we are at 80% of capacity for a given peer. Also not tested, and includes some refactoring. REVIEW REQUESTED! In theory this should mean the network should behave significantly better, because we can control load without actually having to reject requests and hence misroute and backoff. Thanks Ian.

FOR REVIEW.
True in general but the surrounding code does more, will catch it if it's disconnected won't it?