Add a pauseRx(<feedId>, isPaused) method.

[pietgeursen]

According to @dominictarr using request(<feedId>, false) is not a good way to pause replication of a feed.

Context from this ssb message: "On that note, I've just realized that you can't just flipflop replicate.request, because when you do replicate.request(id, false) it will send a "I don't want id signal" (-1) and that will screw with with request-skipping, because if you disconnect with the remote during the -1 state, they will remember that you don't want that feed and not mention it next time. (this may break eventual consistency - there are loads of tests in ebt to catch things like this)

Instead, we want to send a the signal that we do what that feed, but not right now! This is already sent if you are getting it faster from another peer. See the code in epidemic-broadcast-trees/v3.js (and v2.js was the old way to encode that). for example, epidemic-broadcast-trees/events.js timeout (line 390) checks if we are expecting messages on a feed but havn't got them, and cycles to another peer for that feed. it calls setNotes(peer, feed, seq, rx) to tell peer that we do (rx=true) or do not (rx=false) want to receive feed from them, but that we are up to seq.

To pause a feed, we just need to be able to set all peers to rx=false for a given feed."