Indexed Scripts/Templates: Small fix.

Throw exception when/if private ctor is called.

See #5921
Indexed Scripts/Templates: Refactor.

This commit creates two new base classes.
DelegatingActionListener
This class is used when a transport action wants to issue a new request but delagate the responses to its Listener.
HandledTransportAction
This class is implements the common pattern of subclassing BaseTransportRequestHandler and registering the handler into the
TransportService.

See #5921