RFR 8161474: Extract interface from java.net.http.RawChannel

[Pavel Rappo]

Hello,

Could you please review the following change for [1]?

    http://cr.openjdk.java.net/~prappo/8161474/webrev.00/

This change is purely a refactoring. No functionality changes are intended.

RawChannel is an I/O abstraction that represents a WebSocket implementation's
view on the "network+server" system. Extracting an interface from this class
will allow to provide a high-level quality gray-box tests as virtually any event
from the network and/or server could be emulated by an appropriate
implementation of RawChannel.

Thanks,
-Pavel

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[1] https://bugs.openjdk.java.net/browse/JDK-8161474