RFR(S): 8230126: delay_to_keep_mmu can delay shutdown

[Kim Barrett]

Please review this change to G1ConcurrentMarkThread::delay_to_keep_mmu
to use a wait-with-timeout on CGC_lock rather than os::sleep to
implement the delay.  This allows the delay to be terminated early by
a thread termination request.

Also fixed a units bug in the calculation of the delay.  The call to
G1MMUTracker::when_ms was with "now" in seconds and "prediction" in
milliseconds.  This function expects both arguments to be in seconds,
and returns a millisecond result.  This bug was largely masked by the
calculation first clipping the prediction value to the max GC time.

CR:
https://bugs.openjdk.java.net/browse/JDK-8230126

Webrev:
http://cr.openjdk.java.net/~kbarrett/8230126/open.00/

Testing:
mach5 tier1-3

Locally (linux-x64) ran gcbasher test with some additional logging
added to the changed functions to verify the various values looked
okay.