Invites to local users from remote server do not get sent down sync of other local users in the room (sometimes).

This issue has been migrated from [#8605](https://github.com/matrix-org/synapse/issues/8605).

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If `@alice:local` and `@bob:remote` are in a room and `@bob:remote` invites `@charlie:local`, then `@alice:local` may not see the invite come down sync.

This is because the remote server sends an invite over `/invite`, which the local server persists as an outlier and forwards to Charlie. Alice does a `/sync`, the invite does not get returned as it is an outlier. The remote server then sends the invite to the local server in a standard federation transaction, which then flips the outlier bit in the DB. When Alice does another `/sync` they still won't get the event as their since token will be after the invite event.