Update documentation of ConsensusType in protos/orderer/configuration.proto
to reflect implementation.

 - spell out permitted type strings: "solo" / "kafka" / "etcdraft"
 - update migration_state for which messages are permitted on system / standard channels
 - update migration_context for what is required on each migration_state
 - make protos

Change-Id: Ia27d9cd162fe6656fd2bd56ceaf5aae8a6fe5222
Signed-off-by: Yoav Tock <tock@il.ibm.com>