Can be disabled with system property:
 "org.eclipse.jdt.model_save_threads=0"
or tuned with any value > 1.

Saving lot of projects is faster.
Especially ~ twice as fast when files are compressed - which is not I/O
bound but single threaded CPU work.

Change-Id: I8157a374a618715c3a801f30b49854e3a3d03a31
Signed-off-by: Joerg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/188904
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-by: Gayan Perera <gayanper@gmail.com>
Tested-by: JDT Bot <jdt-bot@eclipse.org>