SurrogateGossiper can accumulate infinite amounts of data

[bboreham]

Original message from weaveworks/weave#1763:

"SurrogateGossiper does not (and cannot) know how to merge GossipData, so instead it just stashes it in a queue. That is potentially unbounded since downstream connections in gossip propagation may not be able to accept data at the same rate it is received."

Note that #66 ameliorated this, in the case that some messages are exact duplicates.