RFR: 8241001: Improve logging in the ConcurrentGCBreakpoint mechanism

[Kim Barrett]

Please review this change to the logging output produced by the
ConcurrentGCBreakpoint facility.

Most of its log messages provide low-level information for someone
debugging the breakpoint mechanism or a test using it, with relatively
cryptic messages unless that is what one is doing.  As such, putting
these messages at the debug level generates too much uninformative
clutter for the fairly common case of gc*=debug logging; those
messages have been demoted to the trace level.

There were logging messages for the concurrent cycle start
(idle->active) and end (active->idle) transitions, which were
duplicative of other logging messages from the normal concurrent cycle
operation.  Those logging messages have been removed.  However, the
message for the active->idle transition when there is an active run-to
request has been retained, but the message text has been improved.

gc+breakpoint logging has also been configured to produce the normal
"GC(n)" prefix, for those log messages produced by the collector.
User requests such as run-to don't (and can't) have that prefix, as
they don't occur within the context of the collection but instead come
from Java threads outside the collector.

CR:
https://bugs.openjdk.java.net/browse/JDK-8241001

Webrev:
https://cr.openjdk.java.net/~kbarrett/8241001/open.00/

Testing:
Locally (linux-x64) ran TestConcurrentGCBreakpoints.java with various
logging options and verified the results were as described.