Intelligent block propagation

[fluffypony]

Right now blocks are broadcast to all peers, regardless of whether they already have that block.

Peers should be sent a hash of the new block and should respond confirming whether or not they want to accept the block. Obviously if the block matching that hash has already been received from a connected peer, it does not need to be sent.

[iamsmooth]

Want to avoid slowing propagation too much though, so this gets tricky. The current algorithm is probably close to optimal until blocks get larger