Indexed DB can have inconsistency in membership event content

It's possible for the sync store to have incorrect content of a m.room.member event in roomsData.join.$roomID.state[*], but correct content for that same event in roomsData.join.$roomID.timeline[*].

To reproduce:

• Visit develop.element.io in two tabs, each tab signed in with a different user
• Create/visit a room that has both users in it
• Have one user leave the room
• With the user that remains in the room:
  • Open Element devtools and view Explore room state > m.room.member > MXID of user who left
  • Open browser devtools and view Indexed DB for https://develop.element.io > matrix-js-sdk:riot-web-sync > sync
• See that Element devtools shows the correct value of content.membership = "leave"
• See that the Indexed DB has the same value for content.membership in the timeline event for the same eventID
• See that the Indexed DB has an *incorrect value of content.membership = "join" in the state event for the same eventID

With that said, any client that uses timeline to load state won't have any problems (and I assume Element Web does that, as it has accurate member lists in rooms). But if a client were read from state, it would get incorrect data.

Start by tracing how the Indexed DB sync store receives roomsData.join.$roomID.state[*] and timeline[*] events, using the two-user reproduction described in the issue. Compare the stored m.room.member event content for the same event ID in both paths; done means both entries preserve the correct membership value, such as leave.