Improve output of discovered/cleared references during reference processing

Currently after JDK-8202845 we print Discovered and Cleared references for every reference type after reference processing in a very verbose way:

   SoftReference:
     Discovered: 0
     Cleared: 0
   WeakReference:
     Discovered: 30
     Cleared: 30
   FinalReference:
     Discovered: 0
     Cleared: 0
   PhantomReference:
     Discovered: 5
     Cleared: 5
I.e. there is not much information on every line.

Further issues:
- this is printed using gc+phases+ref log tags. This information is not really related to gc phases; it would be nicer to have some output with gc+ref too (which jdk9 has).
- it would also be nice to print the number of encountered refs (during discovery) and instead of the number of Cleared refs (or in addition) the number of Enqueued ones.