Crypto: key reshare does not work if there was a device change

Steps to reproduce:
- Alice and Bob with devices Alice1 and Bob1 are in an encrypted room
- Alice sends a message
- Bob logs in with device Bob2
- If Bob1 makes a reshare request to Alice1 for the key of the message, Alice1 will reject it with the error `[MXMegolmEncryption] reshareKey: ERROR: Never shared megolm key with this device`

There is an issue in the implementation of the crypto store at https://github.com/matrix-org/matrix-ios-sdk/blob/v0.19.8/MatrixSDK/Crypto/Data/Store/MXRealmCryptoStore/MXRealmCryptoStore.m#L646.

With this "delete all" action, all existing references to those `MXRealmDeviceInfo` objects are set to null by RealmDB.
So, a `MXRealmSharedOutboundSession` object that depends on it will be reset like this:
```
MXRealmSharedOutboundSession {
roomId = !ROOwNxCLjNICmOoaMy:localhost:8480;
sessionId = xQVT9qTT4yVurxg8bgHqz9rHz1So/jetZ4FgLlf1C6Q;
device = (null);
messageIndex = 0;
}
```

Next time we will try to check if we already shared a session toBob1 by querying `MXRealmSharedOutboundSession` objects by device id at this [line](MXRealmSharedOutboundSession), we will find nothing.

Start in MatrixSDK/Crypto/Data/Store/MXRealmCryptoStore/MXRealmCryptoStore.m around line 646 and inspect how the delete-all action affects MXRealmDeviceInfo and MXRealmSharedOutboundSession references. Reproduce the Alice1/Bob1/Bob2 device-change scenario and verify that Bob1 can receive a reshare request for the existing message key after the device change.

Written by the indexing model from the issue text.