Add support to CryptoStores for migration to Rust Crypto

[richvdh]

To support migration to Rust Crypto, I think we need to add a bunch of methods to the legacy CryptoStore implementations (all three of them, though MemoryStore will be a no-op):

• containsData: unlike the other methods, needs to be callable before startup. Checks if the database might contain data that needs migration (probably via the existing exists static methods).
• get/setMigrationState: stores/returns some integer which represents how many migration steps we have done (see Rust crypto: support migration to Rust crypto #3964). We can probably stash the data itself in the account object.
• getEndToEndSessionsBatch, getEndToEndInboundGroupSessionsBatch: retrieve a batch of (pickled) Olm/Megolm sessions. getEndToEndInboundGroupSessionsBatch also needs to check sessions_needing_backup to see which sessions still need backup.
• deleteEndToEndSessionsBatch, deleteEndToEndInboundGroupSessionsBatch: delete a batch of Olm/Megolm sessions. (Presumably we pass them a list of sessions?)