Prequisites for ARI Outbound Websockets

Prequisites for ARI Outbound Websockets

stasis:
* Added stasis_app_is_registered().
* Added stasis_app_control_mark_failed().
* Added stasis_app_control_is_failed().
* Fixed res_stasis_device_state so unsubscribe all works properly.
* Modified stasis_app_unregister() to unsubscribe from all event sources.
* Modified stasis_app_exec to return -1 if stasis_app_control_is_failed()
  returns true.

http:
* Added ast_http_create_basic_auth_header().

md5:
* Added define for MD5_DIGEST_LENGTH.

tcptls:
* Added flag to ast_tcptls_session_args to suppress connection log messages
  to give callers more control over logging.

http_websocket:
* Add flag to ast_websocket_client_options to suppress connection log messages
  to give callers more control over logging.
* Added username and password to ast_websocket_client_options to support
  outbound basic authentication.
* Added ast_websocket_result_to_str().