Sync accumulator should be split per room.

### Steps to reproduce

At the risk of stating the obvious: there is no reason for the sync accumulator to maintain one gigantic JSON blob of all your accumulated sync response; it could split it per room and so drastically reduce memory and IO churn when it needs to be updated. This would reduce RAM significantly, and would be a fairly quick fix (similar to https://github.com/matrix-org/matrix-js-sdk/pull/5095).

### Outcome

#### What did you expect?

Sensible resource usage.

### Operating system

_No response_

### Application version

_No response_

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

No