This change prevents internal state from leaking to the caller
through *Member pointers. Also, cluster simulations now show a nice
data propagation latency graph.

This change allows seamless node restarts without having to keep saved
state (the other nodes will share the full list of members on first
contact anyway, and the new node heartbeats won't be considered stale).

This commit breaks the API (NewGossip -> New), but improves the
overall correctness dramatically (including various bug fixes), also
thanks to the decent test suite.