Add support to Tornado 6

[nicholasamorim]

Tornado 6 does not contain the StackContext used in the current implementation.

How could we support Tornado 6? Should one of the already-implemented ScopeManagers be used?

Should the contextvars module be used? That would limit the usage to Python 3.7+ though.

[condorcet]

Because Tornado 6 fully asyncio-based framework, I think it's better to use AsyncioScopeManager. You can see this PoC about the problem opentracing/opentracing-python#118

[Alessandro-Barbieri]

Any progress?

[matej14086]

Any progress?

[yurishkuro]

See #118