Create indexes for foreign keys in 3.1 branch

Many users are using jBPM 3.1.x and do not have the ability to perform a major upgrade. However 3.1.x suffers from a significant problem in that many foreign keys do not have indexes (this problem was fixed for 3.2.0). This causes queries against these keys to scan the full table. In the case of database which use row locks for both reads and writes, this can lead to deadlock scenarios because many, many rows are locked unnecessarily. Tables are frequently queried by their foreign keys; that's why they exist. Therefore these columns should be indexed.

(Relates to JBPM-735)