Support blocking RemoteService interfaces

[niloc132]

GWT2's Generator based RPC implementation encouraged two interfaces, blocking and async, where the async interface could be generated at design time (as GWT generators run too late to allow developers to reference them).

This issue tracks two features:

• Allow the developer to author only the RemoteService blocking interface, and have the async interface generated at compile time, so it can be used in other Java code
• Emit adaptor code which makes it possible to implement the blocking interface, but have it be called by the async version (with appropriate wrapping try/catch blocks).

One issue that needs to be tracked: the Callback interface prefers some kind of error object (both to avoid declaring Throwable and getting the whole type tree, and to avoid the potential security leak of sending stack traces to clients) - either support will need to be added to handle the union of throws args, or some adapter to let exceptions be turned into error objects.