`filewriter`: deployment topology

We need to define how the filewriter and other cooperating processes will run.

- **One-per-instrument**: each FW process incrementally writing out the events from a single instrument
- **Pooled**: some number, perhaps dynamic, of filewriters which may pick up a writing job from any instrument. This is the architecture that ESS use.

Things to consider:
- **Pooled architecture** is a requirement if we might ever collect data faster than it can be written to disk. This seems unlikely at present.
- Consider interaction with https://github.com/ISISComputingGroup/DataStreaming/issues/96 ; this likely makes a pooled architecture more desirable
- Needs to work for HRPD-X/SANDALS2/WISH/VESUVIO etc before MNeuData is available. We do not need to _solve_ the MNeuData case in this issue, but we should pick an architecture which makes it _possible_ to extend in likely MNeuData directions later.
- Consider interaction with https://github.com/ISISComputingGroup/DataStreaming/issues/85

Start by reading the deployment-topology alternatives in this issue, then review linked issues #96 and #85 for related constraints. Identify how the chosen architecture must support HRPD-X, SANDALS2, WISH, and VESUVIO while remaining extensible toward MNeuData; done means the architecture is agreed and documented.

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