SSP-2305 Per-batch Hibernate sessions for mail send job

Conflicts:
	src/main/java/org/jasig/ssp/service/impl/ScheduledTaskWrapperServiceImpl.java

This was a cherry-pick of 337e648d2cf41ca253bc2315d57a9555067b9b01 from
rel-2-2-patches. The conflicts were trivial to resolve and were caused
mainly by the target class having been previously modified to accept
a task name argument whenever executing a task.