admin.StopRPC added to console

[bas-vk]

Adds support for stopping the rpc service in the console, this closes #729.

1. When the user issues a stopRPC command the listener is stopped immediately and new connections are not accepted.
2. Active connections (http keep-alive) follow the go principe and run in their own goroutines. The net package doesn't provide an easy way to interact with these routines. Therefore the ClosableConnection type was added. This is a net.TCPConn but with a specialised Read method. This method tests if the listener was stopped, in that case it returns an io.EOF. If not stopped it cals the TCPConn.Read method.
3. The net package would still call the handler in case of an read error before it stops. Therefore the NewStoppableHandler is added. This wraps the default json/cors handlers and does a check if the listener was stopped. In that case it would return an error indicating that the RPC service is stopped. After that the connection is closed.

Note, it doesn't do a graceful shutdown. This could be added but will stall the console while waiting for idle connections to timeout.

[fjl]

As general feedback, there is no need to store pointers to channels. Just store the channel directly.

[fjl]

What exactly is so bad about just closing the listener and letting those connections time out? Given that stopRPC isn't something most people will ever use, it doesn't really warrant the huge complexity.

[bas-vk]

Not closing connections can cause unexpected behaviour from the users point of view. If you let these connections remain open users will be able to execute transactions as long as the connection is kept alive which is not really what you would expected after a stopRPC.

Closing idle connection will (hopefully) be supported in go 1.5. What I suggest, support closing idle connections for now and when golang/go/issues/4674 makes it to go 1.5 and most of our users are using go 1.5 replace this code with the code from the package.

[bas-vk]

Just had a quick chat with @obscuren, we use the PR as is and when the CloseIdeConnections is implemented in the net package we remove this code. I will make add a comment that this code should be removed in the future.

[fjl]

                                                                                  Maybe it should not be exported.

[zelig]

this can merge if green