Support async Okhttp client requests

[eyalkoren]

hello @eyalkoren , can you give some comments on my thoughts for implementing this:
as I understand, we have Request with enqueue method, that gets Callback object as an argument.
So after exiting enqueue method, we should add instrumentation to new Thread and check for response or error in onFailure, onResponse methods respectively.
And another way after exiting, we should check for call of Request.cancel() method and instrument it.

[eyalkoren]

When monitoring async http calls, there are several things we need to address:

1. get and fill span data when coming from both involved threads properly, where properly means that span's state can only be changed before being closed. This can be tricky as sometimes spans can be opened by one thread and closed by another thread.
2. make sure we inject the right trace context into the request, so that distributed tracing works properly
3. make sure span lifecycle is managed properly, which means- starting the span once and closing the span exactly once
4. whenever a span is activated on a thread (not always required)- it must be deactivated properly

You addressed (1) and somewhat (3). We are currently adding an infrastructure that will enable us tracing async operations, so this is a bit premature, but it will soon be in and if you want to get an idea of what it would look like, you can see #430