Synapse only maintains a threepid's relationship with zero or one identity servers, but I think it is (theoretically) possible for it one threepid to be associated with many identity servers

This issue has been migrated from [#5539](https://github.com/matrix-org/synapse/issues/5539).

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`@bob:homeserver.computer` could:
- fire up a Riot with the IS `alpha.im`
- publish the threepid `bob@gmail.com` to `alpha.im`
- fire up another Riot with the IS `beta.im`
- publish `bob@gmail.com` to `beta.im`

Then I expect we'd lose the assocation with `alpha.im`, meaning the HS could no longer tidy up the threepid binding on account close.