Internal: made it possible to disable the main transport handler in TransportShardSingleOperationAction

TransportShardSingleOperationAction is currently subclassed by different transport actions. Some of them are internal only, meaning that their execution will take place only in the same node where their parent execution took place. That means that their main transport handler doesn't need to be registered, the only transport handler that's needed is the shard level one.

Added `isSubAction` method (defaults to false) to the parent class that tells whether the action is a main one or a subaction, used to decide whether we need to register the main transport handler.

Closes #7285
Internal: auto create index to keep around headers and context of the request that caused it

Closes #7331