add isLoggable() check to doLog()

Moving the mouse pointer over a java.awt.Window using JRE7 consumes
aprox. twice the CPU time as with JRE6 when logging is used (when a call
to java.util.logging.getLogger(String)---even without using the returned
Logger--- has been done). Keyboard key event processing suffers the same
performance penalty. For steps to reproduce this problem please look at:
http://www.java.net/forum/topic/performance/general-performance-discussion/jre7s-javaawt-performance-degradation-caused-javautillogging-linux
.

The performance degradation seems to be "enabled" by the call of
sun.util.logging.PlatformLogger.redirectPlatformLoggers() done inside
java.util.LogManager.readPrimorialConfiguration() on JRE7. Call chain:
Logger.getLogger()->LogManager.getLogManager()->
LogManager.readPrimordialConfiguration()->sun.util.logging.PlatformLogger.redirectPlatformLoggers().

This call is not made on JRE6. If this problem is not easy to solve then
I guess is better to remove this call from JRE7's
readPrimordialConfiguration as it does more harm than good.

Please let me know if there is a better place to report this problem. A
couple of weeks ago I "opened" a bug report on bugs.sun.com but I
received no response at all.

Reported on jdk7u-dev@:
http://mail.openjdk.java.net/pipermail/jdk7u-dev/2012-April/002667.html