RFR: Pacer for evacuation should print "Avail" to capture discounting

[Aleksey Shipilev]

Pacer for Evacuation prints "Free" for the value that is actually free minus cset, because it
discounts cset for future evacs. Makes the log misleading, better to say "Avail":

diff -r 2ce3f06bf1ff src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp       Thu Jul 12 12:19:26 2018 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp       Thu Jul 12 12:20:42 2018 +0200
@@ -100,7 +100,7 @@

   restart_with(non_taxable, tax);

-  log_info(gc, ergo)("Pacer for Evacuation. Used CSet: " SIZE_FORMAT "M, Free: " SIZE_FORMAT
+  log_info(gc, ergo)("Pacer for Evacuation. Used CSet: " SIZE_FORMAT "M, Avail: " SIZE_FORMAT
                      "M, Non-Taxable: " SIZE_FORMAT "M, Alloc Tax Rate: %.1fx",
                      used / M, free / M, non_taxable / M, tax);
 }

Testing: eyeballing logs

-Aleksey