Mirroring states.
No new value is expected to be added in future.
Values:
MIRROR_STATE_UNSPECIFIED (0):
Unspecified MirrorState
PREPARING (1):
Destination volume is being prepared.
MIRRORED (2):
Destination volume has been initialized and
is ready to receive replication transfers.
STOPPED (3):
Destination volume is not receiving
replication transfers.
TRANSFERRING (4):
Replication is in progress.