Supress membership field on MSC4115

[kegsay]

Part of element-hq/element-meta#2317

Servers that support MSC4115 will be sending down a new key unsigned.membership that represents:

the membership of the user making the request, according to the state of the room at the time of the event being returned

This is incompatible with sliding sync's current architecture, which deduplicates events in the events table. This means Alice could receive the event first, with unsigned.membership representing Alice's membership, and then we could serve that same event to Bob. This results in sliding sync lying about this value to clients. We should ensure that we strip this key for now.

Longer term, we need to add a new table to track this membership value for each user for each event and then repopulate it when the user connects to the proxy.