Deprecate these options in favor of Unified logging option

-Xlog:safepoint+stats

Since PrintSafepointStatistics flag is a product mode flags, and maybe used by customers, for JDK 10 it would be accepted and aliased to the logging flags, with a message (like TraceExceptions).
PrintSafepointStatisticsCount controls how many lines of safepoint statistics are buffered before printing them out at safepoint end. PrintSafepointStatisticsTimeout, if greater than zero, turns on PrintSafepointStatistics but does not buffer the output. Both of these options would become obsolete.

% java -XX:+PrintSafepointStatistics -version
[0.000s][warning][arguments] -XX:+PrintSafepointStatistics is deprecated. Will use -Xlog:safepoint+stats=debug instead.
java version "10-internal"
Java(TM) SE Runtime Environment (fastdebug build 10-internal+0-2017-11-13-2209279.cphillim.10hotspot-all)
Java HotSpot(TM) 64-Bit Server VM (fastdebug build 10-internal+0-2017-11-13-2209279.cphillim.10hotspot-all, mixed mode)

Only knowledgeable users would use these options and can figure out how to replace them with -Xlog. Most of the other logging options they would be interested in related to safepoint time have already been moved to Unified Logging.