`process_sync_changes` always updates `account` in the store

Every time `process_sync_changes` is called, it persists the current state of the Account to the store (https://github.com/matrix-org/matrix-rust-sdk/blob/8deb0ff2e18c33d3588eef78e6040fe8a4d4cf41/crates/matrix-sdk-crypto/src/machine.rs#L1171-L1174).

IndexedDB storage operations can be quite slow, and since `process_sync_changes` is called for every sync (indeed, three times for every sync on Web), this is quite expensive and can be rather slow.

I don't really follow why we need to store the account at this point (why not when we create the session?), but could we give it a 'dirty' bit to indicate whether it needs updating, rather than doing it unconditionally?

Start in crates/matrix-sdk-crypto/src/machine.rs at process_sync_changes and inspect the account persistence around lines 1171-1174. Trace when the Account changes and how IndexedDB storage is used on Web; done means unchanged accounts are not persisted on every sync while necessary updates still reach the store.

Written by the indexing model from the issue text.