This patch allows more network topologies, reducing the number of
required connections. The used protocol is a simple relaying (gossip)
scheme.

* Endorsements are directly relayed, no matter if there were already
sent ;
* Nodes can announce that they have some spores *without* sending the
whole payload, using Gossip call function ;
* This will both save bandwidth while enabling NAT-ed clients to fully
take part in SporeDB networks ;
* Spores are cached in a LRU cache to bound memory consumption
over-time, while providing an efficient data-structure.

The previous modifications do not break SporeDB P2P protocol
backward-compatibility.