Add PARENT_CLOSE_POLICY_DISCONNECTED

[mfateev]

Is your feature request related to a problem? Please describe.

It is not possible to start child workflows in a fully disconnected mode which will not report completion/failure to the parent. It causes additional performance overhead as well as UnhandledCommand without any benefit:

Describe the solution you'd like

Add DISCONNECTED to the ParentClosePolicy. When a child is created with this policy, only its start (through ChildWorkflowExecutionStartedEvent) is reported to the parent. The child's completion and failure are not reported. The parent's completion doesn't affect the child as well.