etcdserverpb, v3rpc: add Snapshot to Maintenance RPC service

etcdserver/etcdserverpb/rpc.proto

@@ -85,6 +85,9 @@ service Maintenance {
   // This is designed for testing; do not use this in production when there
   // are ongoing transactions.
   rpc Hash(HashRequest) returns (HashResponse) {}
+
+  // Snapshot sends a snapshot of the entire backend
+  rpc Snapshot(SnapshotRequest) returns (stream SnapshotResponse) {}
 }
 
 service Auth {
@@ -311,6 +314,21 @@ message HashResponse {
   uint32 hash = 2;
 }
 
+message SnapshotRequest {
+}
+
+message SnapshotResponse {
+  // header has the current store information. The first header in the snapshot
+  // stream indicates the point in time of the snapshot.
+  ResponseHeader header = 1;
+
+  // remaining_bytes is the number of blob bytes to be sent after this message
+  uint64 remaining_bytes = 2;
+
+  // blob has the next chunk of the snapshot in the snapshot stream.
+  bytes blob = 3;
+}
+
 message WatchRequest {
   oneof request_union {
     WatchCreateRequest create_request = 1;