Media over Websocket Channel Driver

* Created chan_websocket which can exchange media over both inbound and
outbound websockets which the driver will frame and time.
See http://s.asterisk.net/mow for more information.

* res_http_websocket: Made defines for max message size public and converted
a few nuisance verbose messages to debugs.

* main/channel.c: Changed an obsolete nuisance error to a debug.

* ARI channels: Updated externalMedia to include chan_websocket as a supported
transport.

UserNote: A new channel driver "chan_websocket" is now available. It can
exchange media over both inbound and outbound websockets and will both frame
and re-time the media it receives.
See http://s.asterisk.net/mow for more information.

UserNote: The ARI channels/externalMedia API now includes support for the
WebSocket transport provided by chan_websocket.

(cherry picked from commit 07fd3af897)

include/asterisk/http_websocket.h

@@ -77,6 +77,14 @@ enum ast_websocket_opcode {
 	AST_WEBSOCKET_OPCODE_CONTINUATION = 0x0, /*!< Continuation of a previous frame */
 };
 
+#ifdef LOW_MEMORY
+/*! \brief Size of the pre-determined buffer for WebSocket frames */
+#define AST_WEBSOCKET_MAX_RX_PAYLOAD_SIZE 8192
+#else
+/*! \brief Size of the pre-determined buffer for WebSocket frames */
+#define AST_WEBSOCKET_MAX_RX_PAYLOAD_SIZE 65535
+#endif
+
 /*!
  * \brief Opaque structure for WebSocket server.
  * \since 12