use consistent worker IDs for synapse stream writers

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Is your feature request related to a problem? Please describe.
When I update my stream writer worker conf, the IDs get mixed up, messing up my metrics.

Describe the solution you'd like
Rather than all of the stream writer types counting towards the stream writer IDs, they should be treated like individual worker types. Then 2 events stream writers will have IDs 0 and 1, and a third stream writer for presence will also have ID 0.

If I then (e.g.) reduce the events stream writers to 1, the IDs will be stable.

Describe alternatives you've considered
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Additional context
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