Saga with both IAmStartedByMessages and IHandleTimeouts for same message causes timeout to fire for empty saga

[SimonCropp]

Messages handled by a saga that contains a explicit saga id should not be allowed to create a new saga if the given saga can't be found in storage. The saga loader has now been modified to check for the existence of the saga id header and also check that the message is directed to the specific saga by comparing the saga type with the saga type header.

To verify

public class MySaga : Saga<MySagaData>,
    IAmStartedByMessages<SagaInitiateMessage>,
    IAmStartedByMessages<MyTimeout>,
    IHandleTimeouts<MyTimeout>
{
    public void Handle(SagaInitiateMessage message)
    {
        Data.MyProp = 100;
        RequestTimeout<MyTimeout>(TimeSpan.FromSeconds(25));
        //after this delete the saga data from raven but dont delete the timeout
    }

    public void Timeout(MyTimeout state)
    {
        var dictionary = Bus.CurrentMessageContext.Headers;
        // the timeout will fire but the saga data will be empty
        // have a look in the headers. the timeout is associated with an exiting saga. ie no this one
        //Note the MyProp value here will be 0 and the id will be a new guid
        Debug.WriteLine(Data.MyProp);
    }

    public void Handle(MyTimeout message)
    {
        Debug.WriteLine(Data.MyProp);
    }
}

working repro here https://github.com/Particular/BugsRepro/tree/master/TimeoutAfterComplete

[indualagarsamy]

This scenario should invoke the saga not found, instead of trying to create a new saga.

[andreasohlund]

@indualagarsamy please verify and start the release + backport procedure