Move HDC (Hippo Diagnostics Context) API to hippo-cms7-common

he HDC API and its monitoring feature [1] is really useful even in production (thanks to threshold and depth settings).
Sometimes I also want to monitor CMS applications to find the bottlenecks in non-local environment. Sometimes you cannot easily reproduce that kind of performance issue in local env.

So, I think the HDC API should be shared even in non-HST applications because it has been successfully used already.
It will be great if we move the HDC API to hippo-cms7-commons library.

By the way the current HDC is in a 'hst' package (org.hippoecm.hst.diagnosis).
It should be fine fine with copying it to the shared lib into a new (common) package and have the HST HDC classes extend the classes in the new common package (and deprecate the HST HDC).

[1] http://www.onehippo.org/library/concepts/request-handling/hst-page-diagnostics.html