update vendor

Signed-off-by: Jess Frazelle <acidburn@microsoft.com>

vendor/google.golang.org/grpc/Documentation/concurrency.md

@@ -0,0 +1,33 @@
+# Concurrency
+
+In general, gRPC-go provides a concurrency-friendly API. What follows are some
+guidelines.
+
+## Clients
+
+A [ClientConn][client-conn] can safely be accessed concurrently. Using
+[helloworld][helloworld] as an example, one could share the `ClientConn` across
+multiple goroutines to create multiple `GreeterClient` types. In this case, RPCs
+would be sent in parallel.
+
+## Streams
+
+When using streams, one must take care to avoid calling either `SendMsg` or
+`RecvMsg` multiple times against the same [Stream][stream] from different
+goroutines. In other words, it's safe to have a goroutine calling `SendMsg` and
+another goroutine calling `RecvMsg` on the same stream at the same time. But it
+is not safe to call `SendMsg` on the same stream in different goroutines, or to
+call `RecvMsg` on the same stream in different goroutines.
+
+## Servers
+
+Each RPC handler attached to a registered server will be invoked in its own
+goroutine. For example, [SayHello][say-hello] will be invoked in its own
+goroutine. The same is true for service handlers for streaming RPCs, as seen
+in the route guide example [here][route-guide-stream].
+
+[helloworld]: https://github.com/grpc/grpc-go/blob/master/examples/helloworld/greeter_client/main.go#L43
+[client-conn]: https://godoc.org/google.golang.org/grpc#ClientConn
+[stream]: https://godoc.org/google.golang.org/grpc#Stream
+[say-hello]: https://github.com/grpc/grpc-go/blob/master/examples/helloworld/greeter_server/main.go#L41
+[route-guide-stream]: https://github.com/grpc/grpc-go/blob/master/examples/route_guide/server/server.go#L126