Change the default strategy from Singleton to PerProcessInstance

Singleton runtime strategy has some downsides:

• Low performance under load
• Issue with JTA transaction https://access.redhat.com/solutions/3462931
• Issue with EJBTimerScheduler https://access.redhat.com/solutions/4065521

So we needed to suggest customers to move from Singleton to PerProcessInstance at the later stage of development (or even in production). Changing the default to PerProcessInstance would decrease such problems.

Places to be changed would likely be:

• KModuleDeploymentUnit.strategy's default value
• kie-deployment-descriptor.xml generated by business-central

But note that we need to care about backward compatibility. If you think it doesn't fit for minor version up, let's postpone to RHPAM 8. At that time, we may mark Singleton as "deprecated".