[FAB-11918] Add raft index to block metadata

This CR adds index of etcd/raft entry to be block metadata so
that we know where to continue rafting upon reboot.

Change-Id: If6388b7612741fbc4fa31c95881937bd8d2a82cc
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>

Author: guoger

protos/orderer/etcdraft/configuration.pb.go

@@ -38,12 +38,14 @@ message Options {
 
 // RaftMetadata stores data used by the Raft OSNs when
 // coordinating with each other, to be serialized into
-// block meta dta field and used after failres and restarts
+// block meta dta field and used after failres and restarts.
 message RaftMetadata {
     // Maintains a mapping between the cluster's OSNs
     // and their Raft IDs.
     map<uint64, Consenter> consenters = 1;
     // Carries the Raft ID value that will be assigned
     // to the next OSN that will join this cluster.
     uint64 nextConsenterID = 2;
+    // Index of etcd/raft entry for current block.
+    uint64 raftIndex = 3;
 }