Transactions are now propagated to peers from which we have not yet
received the transaction. This will significantly reduce the chatter on
the network.
Moved new mined block handler to the protocol handler and moved
transaction handling to protocol handler.
* Improved block propagation by sending blocks only to peers to which, as
far as we know, the peer does not know about.
* Made sub protocol its own manager
* SubProtocol now contains the p2p.Protocol which is used instead of
a function-returning-protocol thing.
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