Try to use refcounting to optimise GC phase

[pshirshov]

We may count references while building plan and apply basic cleanups without running full tracer

[neko-kai]

The idea is to run a first pass over all plan nodes and count the times each DIKey is used as a parameter. This can prune nearly all non-circular cases immediately, leaving less keys for TracingDIGc to process.

A newcomer may take this ticket.

[pshirshov]

I guess this is not relevant anymore since the introduction of SemigraphSolver. The tracer is, essentially, still there (traceGrouped) though I doubt we may achieve any sensible performance gains with additional "ref-counting" pass over the initial input. I'll close this for now.