ari_websockets: Fix two issues in the cleanup of outbound websockets.

ari_websockets: Fix two issues in the cleanup of outbound websockets.

1.  session_cleanup() now saves the websocket type before unlinking the
session from the session registry.  This prevents a FRACK when cleaning
up per-call websockets when MALLOC_DEBUG is used.

2.  session_shutdown_cb() and outbound_sessions_load() now call
pthread_cancel() to cancel the session handler thread to prevent the
thread from continually trying to connect to a server after the
connection config has been removed by a reload.  This required the
thread to use pthread_cleanup_push() to clean up its reference to the
session instead of RAII because RAII destructors don't get run when
pthread_cancel() is used.

Resolves: #1894