Support ContextManager APIs and context stack-style auto propagation.

[wu-sheng]

High-level APIs introduced in this PR. By leveraging the thread local, it will be better easy to use.

APIs

High-Level APIs

High level APIs are targeting convenient usages. These APIs use the ThreadLocal to propagate the context, so users could create span at any moment, and the context will finish automatically once the first created span of this thread stopped.

ContextManager::tracing_entry("op1", Some(&injector), |mut span| {
    // Use span freely in this closure
    // Span's start/end time is set automatically with this closure start/end(s).
    span.tag(Tag::new(String::from("tag1"), String::from("value1")));

    ContextManager::tracing_exit("op2", "127.0.0.1:8080", Some(&extractor), |mut span| {
        span.set_component_id(33);
    });

    ContextManager::tracing_local("op3", |mut span| {});
});

[hanahmily]

LGTM