Minor libdeflate correction

• libdeflate is now compatible with uWS::DEDICATED_DECOMPRESSOR and will be used if (and only if) the WebSocket's negotiation downgraded to equivalent of uWS::SHARED_DECOMPRESSOR (client_no_context_takeover). This will boost performance when possible. Of course, uWS::SHARED_DECOMPRESSOR as decompressor will always use libdeflate.
• In other cases, zlib will be used regardless.