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: Ia8fb16728b99e35c0e36a48091b07839fc4d6fc1
Signed-off-by: Joerg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/189701
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>