Add threshold configuration to FactoryProf

[palkan]

Context

When running FactoryProf (FPROF=1), the output may contain a lot of lines for factories that has been used a few times, thus, making it harder to quickly identify overused factories.

We can safely exclude rarely used factories from the output without sacrificing the value of the profile.

TODO

Let's add a configuration parameter to FactoryProf, threshold, which would be used in the profile printer to ignore factories which total number of calls is less than the provided threshold.

It should be possible to set the threshold value via Ruby or env vars (FPROF_THRESHOLD=N).