WebSocket Server with binary mode support

[hschneider]

Is your feature request related to a problem? Please describe.
Some situations afford to send and receive binary WebSocket messages on an Extension's side.
E.g. when the WebSocket client implementation does not support text-messages.

Describe the solution you'd like
The server should detect incoming text- or binary messages by their opcode.
Then it switches to mode_bin or mode_text for the following conversation.
So if the incoming message is binary, all sent messages to this particular extension will be binary, too.
Other extensions in the same app should still run in their own mode.

Describe alternatives you've considered
No alternatives. If a client-implementation is limited to binary messages, it cannot be used.

Additional context
You can put this on low-priority - it's only a nice-to-have, not a show-stopper :-)

[hschneider]

Looks like this should improve performance, too:
#1122