implement jbpm key generation

mechanism:

1) add JBPM4_PROPERTIES table (See also JBPM-2527)
2) define a property called NEXTID with a long value that is initialized at 1
3) define a KeyGeneratorImpl in the process-engine context, with eager initialization
4) make sure that all hibernate entities during construction get a value from the id generator

• each id generator will acquire a block of ids by updating the NEXTID property
• block acquisition is guarded with optimistic locking
• ids will be consumed from the block (very fast, without db access)
  5) remove all the explicit flushes
  6) implement a DefaultKeyGenerator that only uses in memory key generation in case people work without persistence

public abstract class KeyGenerator {
// similar as ScriptManager
public static KeyGenerator getKeyGenerator()

{...}
// similar as ScriptManager
public static KeyGenerator getDefaultKeyGenerator() {...}

public long getNextId()

{...}

}

Motivations:

1) database portability : see also JBPM-2510
2) performance
JBPM-2510