Dehydrated devices: API change to make paging consistent with other APIs

[andybalaam]

make pagination consistent with rest of Matrix

Will require:

• updating the MSC
• updating the Synapse implementation
• updating Rust SDK
• updating JS-SDK

Re-check breaking backwards compatibility with @mxandreas.

To do:

• Omit next_batch instead of returning empty events array
• Rename the request parameter from instead of next_batch
• (Check that we never delete to-device messages in the latest version of this MSC)

[mxandreas]

Since it is currently not known when this change will be made, breaking the backwards compatibility needs to be checked. It can happen that some users uptake this feature in a production before, and we would then avoid disruptions unless it becomes very expensive.