rpc rewrite: control RPCs using gRPC + separate RPC for data transfer

transport/ssh: update go-netssh to new version
    => supports CloseWrite and Deadlines
    => build: require Go 1.11 (netssh requires it)

.travis.yml

@@ -18,7 +18,7 @@ matrix:
   # all go entries vary only by go version
   - language: go
     go:
-    - "1.10"
+    - "1.11"
     go_import_path: github.com/zrepl/zrepl
     before_install:
         - wget https://github.com/protocolbuffers/protobuf/releases/download/v3.6.1/protoc-3.6.1-linux-x86_64.zip
@@ -36,7 +36,7 @@ matrix:
 
   - language: go
     go:
-    - "1.11"
+    - "master"
     go_import_path: github.com/zrepl/zrepl
     before_install:
         - wget https://github.com/protocolbuffers/protobuf/releases/download/v3.6.1/protoc-3.6.1-linux-x86_64.zip