This implements micropayment payment channels in several parts:
* Adds PaymentChannel[Server|Client]State state machines which
handle initialization of the payment channel, keep track of
basic in-memory state, and check data received from the other
side, based on Mike Hearn's initial implementation.
* StoredPaymentChannel[Client|Server]States manage channel
timeout+broadcasting of relevant transactions at that time,
keeping track of state objects which allow for channel
resume, and are saved/loaded as a WalletExtension.
* Adds PaymentChannel[Client|Server] which manage a connection
by getting new protobufs, generating protobufs for the other
side, properly stepping the associated State object and
ensuring the StoredStates object is properly used to save
state in the wallet.
* Adds PaymentChannel[ClientConnection|ServerListener] which
create TCP sockets to each other and use
PaymentChannel[Client|Server] objects to create/use payment
channels.
The algorithm implemented is the one described at
https://en.bitcoin.it/wiki/Contracts#Example_7:_Rapidly-adjusted_.28micro.29payments_to_a_pre-determined_party
with a slight tweak to use looser SIGHASH flags so that the
Wallet.completeTx code can work its magic by adding more inputs if
it saves on fees.
Thanks to Mike Hearn for the initial state machine implementations
and all his contracts work and Jeremy Spilman for suggesting the
protocol modification that works with non-standard nLockTime
Transactions.
