Implementation of context API to allow for propagation of a limited amount of state through the

call stack and across thread boundaries.

Strongly modeled after the Go context API https://blog.golang.org/context with support for
- cancellation propagation & cancellation listeners
- typed value binding
- timeout/deadline

The major difference with Go is that ThreadLocal is used for propagation instead of parameter
passing as this is considered more idiomatic for Java.

Author: Louis Ryan