Dehydrated devices: does not detect if another client changed the dehydration key in 4S

[uhoreg]

Does not detect when another client saves a new dehydration key in 4S, so when it tries to rotate the dehydrated device, it will continue to use the old dehydration key, which means that it will be undecryptable.

[BillCarsonFr]

Should go in the next batch of work for dehydrated device.

[andybalaam]

This should be fixed if we gossip the dehydration key element-hq/element-meta#2703 , except it would be better if we knew when we had the wrong key, instead of just waiting to be sent a new one.

[andybalaam]

Also element-hq/element-meta#2753 will be part of the fix for this.