Promise?

[lizhanhui]

A promise is a pledge to provide a result at some point in the future. See this document
io.openmessaging.Promise does not fit this semantic. It should be renamed to something like ListenableFuture following naming convention in existing literature and code base: Article, Guava ListenableFuture, JDK Listener, CPP std::future, Folly Future, Twitter future and Boost::future