Add Context-* package and implementation

[mayurkale22]

Add separate packages for Context Propagation/Continues Local Storage (CLS). The primary objective of CLS is to implement a transparent context API, that is, we don't need to pass around a ctx variable everywhere in application code.

In today's (06-19-19) SIG meeting, we have decided to implement generic API, so that it can be used outside OpenTelemetry project.

opentelemetry/context-base
The base implementation that doesn't propagate the context, but handles things like binding.

opentelemetry/context-async-hooks
Implement Async Hooks - async_hooks (since we will be targeting Node 8+).

OpenTracing work on ScopeManager: opentracing/opentracing-javascript#113

OpenCensus work: https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-core/src/internal/cls-ah.ts

[mayurkale22]

Assigned to @vmarchaud as per offline discussion, Thanks for the initiative and help.