Apply shadow checksum

[habuma]

Per the last paragraph in section 4 of Neil Fraser's Differential Synchronization paper...

However, on a network with best-effort delivery, nothing is guaranteed. Therefore a simple checksum of Client Shadow ought to be sent along with the Edits and compared to Server Shadow after the patches have been applied. If the checksum fails to match, then something went wrong and one side or the other must transmit the whole body of the text to get the two parties back in sync. This will result in data loss equal to one synchronization cycle.

Spring Sync should accept a checksum from the client when applying a patch and verify the shadow's state before applying the patch to the local resource. This could be optional, applying the checksum only if one is provided, proceeding without verification otherwise. Or Spring Sync could be configured to fail if the client does not send a checksum.