* get_mutable will search for the latest version of a datum across a set of known devices
* put_mutable will include the device ID in the data it replicates
* delete_mutable will delete from all devices
* datastore inodes will rely on their headers to identify the globally-latest version of the inode data
* user lists and private key indexes will track cross-device versioning so that an update from one device will always be processed by the others.