Implement tracing support

This is a meta issue to discuss how new asyncio tracing API should look like. We will implement it first in uvloop, and later, if necessary, port it to asyncio.

Requirements

1. Base on the new contextvars (PEP 567) API, so that async tasks and callbacks they schedule are traced consistently.

2. Traces should be hierarchical, i.e. it should be possible to see what and how exactly one Task did, what Tasks that it spawned did, etc.

3. Low performance overhead. Ideally we should be able to run tracing in production.

4. Low-level and detailed: we want to be able to measure DNS requests, read/write callbacks performance, throughput, number of callbacks/tasks created etc.

5. Flexibility: traces should be in easy to work with format, so that it's possible to submit them to existing systems like zipkin.

6. Extensibility: the new API should be easy to maintain on both ends: in the event loop and in applications that use it. This means that adding a new kind of trace should not require people to write any glue code.

7. [TBD]

首先审查此 issue 的需求以及 PEP 567 contextvars API；其中未指定任何实现文件、测试或入口点。在开始实现之前，就一个分层、低开销、详细、灵活且可扩展的 asyncio tracing API 达成一致，即视为完成。