FastCS Eiger: Allow the ability to happily stop after X number of frames written

[DominicOram]

In MX DLS we have a use case on the eiger where we:

1. Arm the detector without knowing how many frames we will trigger (we do this by sending a very large number of frames to the eiger and get the file writer to write forever)
2. Whilst the detector is arming (which takes a while) we work out how many frames we want to take (using an optical analysis of the sample)
3. By the time we get to the kickoff we now know how many frames we're going to take
4. After complete we want to wait for the expected number of frames to be written before moving on successfully

Therefore we need some way of either updating the detector with the number of expected frames after we've armed and then getting it to stop itself internally or we need to create a status in the plan to watch that the filewriter has written what we expect.