i04 XRC remaining issues

[olliesilvester]

• Eiger's "set mx settings" needs to be done synchronously to avoid arming issue when doing successive XRC's

-XBPM feedback desired behaviour:

1. Move to calculated transmission
2. Don't pause XBPM feedback during collection
3. Trigger xbpm feedback device before doing the grid scan
4. Don't revert transmission to its previous value afterwards
   We should do this by making another preprocessor similar to transmission_and_xbpm_feedback_for_collection_wrapper