RFR: 8278581: Improve reference processing statistics log output

[Albert Mingkun Yang]

The first commit renames `Cleared` to `Dropped` and added `Processed` in the log output.

The second commit compressed the log output into one line per ref-type. The final log format looks like:


[11.803s][debug][gc,phases,ref  ] GC(0)   SoftReference Discovered: 0, Dropped: 0, Processed: 0
[11.803s][debug][gc,phases,ref  ] GC(0)   WeakReference Discovered: 1147, Dropped: 752, Processed: 395
[11.803s][debug][gc,phases,ref  ] GC(0)   FinalReference Discovered: 0, Dropped: 0, Processed: 0
[11.803s][debug][gc,phases,ref  ] GC(0)   PhantomReference Discovered: 242, Dropped: 2, Processed: 240


Test: hotspot_gc

-------------

Commit messages:
 - oneline
 - rp-log-dropped

Changes: https://git.openjdk.java.net/jdk/pull/6806/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6806&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278581
  Stats: 22 lines in 4 files changed: 8 ins; 3 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6806.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6806/head:pull/6806

PR: https://git.openjdk.java.net/jdk/pull/6806