Add machinery to support external transaction coordinators (initial implementation)
always terminate (either commit or rollback) created transaction to prevent active transactions leaking - fixes tests regression
fixing javadoc issues
adding missing TransactionController.newTask() method lost during TaskFactory refactoring
hiding TaskFactory from public API and moving it to impl. package
Associate ManagementContext, ServiceContext and Transaction with the TransactionController, that now works as a scope enforcer for those three (thus, operations contained in Service/ManagementContext and TransactionController can be performed only with the transactions created by the same transaction controller).