RFR JDK-8157273: Simplify outgoing messages queueing policy in WebSocket API

[Pavel Rappo]

Hi,

Could you please review the following change for JDK-8157273?

http://cr.openjdk.java.net/~prappo/8157273/webrev.01/

This change addresses some WebSocket API refinements and enhancements from [1].

Currently WebSocket API allows only one scheduled but not yet sent message (i.e.
an outstanding send operation) at a time. This design was influenced by
java.nio.channels.AsynchronousChannel:

    * <p> Asynchronous channels are safe for use by multiple concurrent threads.
    * Some channel implementations may support concurrent reading and writing, but
    * may not allow more than one read and one write operation to be outstanding at
    * any given time.

Although it is both easy to understand and implement, this was found to not work
well with WebSocket API.

1. In WebSocket API it's not always easily known (in some modes of operation)
for an app when the previous send operation has completed. Thus maintaining a
non-overlapping sends is a burden on the user.

2. In contract with AsynchronousChannel, WebSocket does not allow "short writes"
(i.e. a situation when a Future returned by the send operation may complete
normally after the message has been written only partially.)
The CompletableFuture in WebSocket completes only after an error or when the
whole message has been written. Therefore, there's no inherent risk in
serializing sends in WebSocket. 
The result on the wire will always be some permutation of messages, and not
their chunks.

Thanks,
-Pavel

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[1] https://bugs.openjdk.java.net/browse/JDK-8155621