BSE: beast-sound-engine.cc: keep Jsonipc::InstanceMap per websocket connection
BSE: beast-sound-engine.cc: keep Jsonipc::InstanceMap per websocket connection

Create and destroy a new InstanceMap per websocketpp::connection_hdl to avoid
global leaks. We still need FinalizationGroup support for object cleanups:
	https://github.com/tc39/proposal-weakrefs

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/beast,swesterfeld/beast,tim-janik/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,swesterfeld/beast,tim-janik/beast