Recovery (disk-mapper) and init (bootstrapper)
will have to work with multiple external KMSes
in the future.

So far the masterSecret was sent to the initial bootstrapper
on init/recovery. With this commit this information is encoded
in the kmsURI that is sent during init.
For recover, the communication with the recoveryserver is
changed. Before a streaming gRPC call was used to
exchanges UUID for measurementSecret and state disk key.
Now a standard gRPC is made that includes the same kmsURI &
storageURI that are sent during init.