* Next attempt to extract common logic from instrumentations.
I tried to reduce code repetition in DB client instrumentations, jdbc and mongo ones. This time I experimented with another approach, different from HttpServerTracer. Namely, I have extracted that common `startSpan` method into `DatabaseClientDecorator` itself. The idea is still the same as before, but I wanted to see if we can migrate by smaller steps, reusing much of the existing code and just incrementally reducing public API.
* Extracted separate Tracer after all
* More explicit call depth handling
* Fix format
* More reusable method overloads
c1c02ac949