add synchronous scope manager

[rochdev]

What does this PR do?

Add synchronous scope manager.

Motivation

Because of the issue with async_hooks, there are cases where it's not possible to track the asynchronous context. However, our auto-instrumentation expects to always be able to get the parent span from the scope manager, which makes the it unusable when scope management is completely disabled. This new scope manager works around this issue by allowing to manually re-activate a span on a synchronous scope to make it available when the integration starts its own span.

For example:

tracer.trace('parent', parent => {
  setImmediate(() => {
    // here `tracer.scope().active() is null instead of `parent`
    // calling for example `http.get` would result in a disconnected trace

    return tracer.scope().activate(parent, () => {
      // tracer.scope().active() is now `parent` so the `http` plugin will have access to it
      return http.get('some-url', res => {
        // here the correct scope is restored by the `http` plugin
      })
    })
  })
})

While the async_hooks issue should be fixed in a few months, in the meantime this allows to continue using auto-instrumentation by manually connecting spans from integrations together. The code is extremely simple so it also doesn't add unnecessary maintenance overhead after the fix will have landed.