JBIDE-17530 : ensure we don't read incompletely downloaded files

Signed-off-by: Fred Bricon <fbricon@gmail.com>
JBIDE-17496 JBIDE-17504: Prevent from parallel loading of same repo

This is a workaround to avoid p2 bug 436378
[JBIDE-17528] 'Widget is disposed' exception in JBoss Central