* API change: TransactionConfidence.Listener now takes a reason enum describing the general class of change.
* Confidence listeners are now invoked in the user code thread as well, thus eliminating any chance of unexpected re-entrancy.
* The wallet batches up confidence changes and executes them all at the end of major operations, avoiding confusing intermediate transitions that could occur in the previous design.
* Much code has been simplified as a result and it's now harder to screw up.
This ensures that when user provided event listeners are invoked, we're not holding any locks at that time and thus event listeners can do whatever they want with no risk of accidental inversions or deadlocks. A utility method is available to wait for all preceding events that were triggered to complete, which is useful for unit tests. Reimplement how balance futures work in order to avoid the wallet registering an event handler on itself, this means you cannot accidentally deadlock yourself by running getBalanceFuture().get() inside an event listener.
Future changes will modify how transaction confidence listeners are run to work the same way, and make other kinds of event listener run in the user code thread as well.
The user code mechanism is usable with any executor, opening up the possibility of automatically relaying event listeners into GUI threads for some kinds of apps.
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.
