This changeset:

1. Defines consenter metadata for the etcd/raft plugin, creates the
associated metadata message factory, and registers it with the metadata
factory map in protos/orderer for protolator to work.

2. Adds supports for etcd/raft configuration to the channel
configuration encoder.

3. Adds an "etcdraft" section to the configtx sample YAML and the
backing config structure.

4. Adds a "SampleDevModeEtcdRaft" profile preset to the configtx sample
YAML. This will be consumed by integration tests.

Change-Id: I7460c89fdb91e68e4217932aeed5ca41a131d47b
Signed-off-by: Kostas Christidis <kostas@christidis.io>