Support Ratpack 1.6

[llinder]

Ratpack 1.6 is out and should include a client interceptor API now. I suspect we will get better milage from using the interceptors and hope that it may also fix some anomalies around client tracing that have been seen under heavy load.

[hyleung]

@llinder cool! From #62 (man, that was a long time ago), there were a few tweaks required to even get up to 1.5.4 but if we can jump straight to 1.6 then let's do that as a first step before #44 - when I first started looking at that issue (when I was using the 1.6 snapshot) I hit up against some issues around stuff executing on non-ratpack-managed threads. Tho'...that was a long time ago so 🤞

[llinder]

Thanks for the heads up on the issues. So far the upgrade has been going ok. I actually started on the client interceptor changes first. My biggest concern right now is backwards compatibility with Ratpack 1.5.x. I guess at worst case we end up with 2 branches, either that or we try to be clever on detecting the version and maintain different code internally. Either way its not going to be the most pleasant I suppose.

Sorry I skipped the 1.5.4 upgrade you started. We have been seeing weird things with client tracing where 2 spans are started on occasion and only one is finished. My hope is that the 1.6 interceptors resolve this... if not then we can raise an issue in Ratpack.

[hyleung]

Moved move this issue over to openzipkin-contrib