Narrow down scheduling rule in JavaDebugOptionsManager.updateBreakpointMessages()

[trancexpress]

See suggestion by @mickaelistria here: eclipse-platform/eclipse.platform#2218 (comment)

The same suggestion can be applied to fix: https://bugs.eclipse.org/bugs/show_bug.cgi?id=540132

Since its better to lock less than to lock more, we should apply this suggestion also to: org.eclipse.jdt.internal.debug.ui.JavaDebugOptionsManager.updateBreakpointMessages(IBreakpoint[])