fix#849
This adds an additional `bool` template parameter to
`websocket::stream`:
* When deflateSupported is `true`, the stream will be capable
of negotiating the permessage-deflate websocket extension per
the configured run-time settings.
* When deflateSupported is `false`, the stream will never negotiate
the permessage-deflate websocket extension. Furthermore, all of the
code necessary for implementing the permessage-deflate extension
will be excluded from function instantiations. The resulting emitted
object code should be smaller.
