[FIXED JENKINS-25759] Avoid consuming too much memory while running validateAntFileMask.

Not fully solved, since the scannedDirs field can still grow to be large, but at least clearing files/dirsNotIncluded.
Also imposing a 5s timeout on the scan regardless of file count, and defining a user-customizable bound.
(cherry picked from commit 534328b2)

Conflicts:
	changelog.html