Change InvalidInput to EndOfFile in several cases
Distinguish between network/decode errors in network message decode

It is now visible that EOF (i.e. peer hung up) is interpreted
as a message decode error. Probably what we want to do is reset
the connection on any error. TODO
Store TxOuts directly in UtxoSet rather than in Boxes

This gives a significant speedup during deserialization since we
don't have to allocate for every output.