RFR: Remove ZStatReference's dropped statistics count

[Per Liden]

This patch removes the "dropped" count from the reference processor and 
instead tracks "discovered" events, so that we have the more natural 
"encountered", "discovered" and "enqueued" counters. Dropped could be 
calculated from discovered - enqueued if needed in the future.

http://cr.openjdk.java.net/~pliden/zgc/remove_zstat_reference_dropped/webrev.0/

/Per