[JENKINS-16215] Avoiding unnecessary downloads in FilePath.installIfNecessaryFrom.

The updated version uses the locally cached timestamp, '.timestamp', to set the
'If-Modified-Since' header. 304 Not Modified responses are dealt with by
aborting the installation.
(cherry picked from commit 35f7653b)

Conflicts:
	core/src/test/java/hudson/FilePathTest.java