Enable async tracking for some non-async frameworks

[tylerbenson]

This will allow tracing of additional work being done inside the servlet context.

Closes #304

[realark]

I think we'll also want to turn off the async propagation when the root span finishes.

if (scope instanceof TraceScope) {
  ((TraceScope) scope).setAsyncPropagation(false);
}

[tylerbenson]

@realark I didn't worry about that because the async propagation flag is on the scope, which is recreated every request and goes away when that scope is closed. If we add the flag to turn it off, it kinda clutters the code.

[realark]

👍 Good point, once the scope is closed the async propagation will finish so it's not required to turn it off.