Allow to configure configure and disable constraints jitting

Prior to Java 8 it was possible to disable constraints jitting by setting the drools.permgenThreshold to 0. Since with Java 8 they removed the PermGen space, is no longer possible to control and disable constraints jitting.

It is required to provide a new option, named drools.jittingThreshold, through which it will be possible to configure after how many interpreted evaluations a constraint should be jitted. Special values for this property will be:

0 -> force immediate synchronous jitting (it's adviced to use this only for testing purposes).

-1 (or any other negative number) -> disable jitting

Default value is 20.