Revisit Http client tracing now that Ratpack has interceptors

[hyleung]

The current implementation for tracing http client requests, while it works, is a bit clunky. Much of the yak shaving was because, at the time, Ratpack didn't have any way of intercepting http client requests/responses.

With ratpack/ratpack#1124, Ratpack has added support for interceptors.

We should revisit the current client tracing implementation to see if we can get rid of some of the extra code.

Note: I tried this a little while ago when that Ratpack PR was first merged but ran into some issues with accessing the context from a non-Ratpack managed thread. This may require a little refactoring to get working properly.

[hyleung]

Just gonna close this since it's dependent on on openzipkin-contrib/brave-ratpack#8 now