[#196] rpc/grpc: Implement Client.Conn method

Implement `Client.Conn` method which returns underlying connection as
`io.Closer`. Method is going to be used for forwarding the connection to
superior clients.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

rpc/grpc/conn.go

@@ -0,0 +1,19 @@
+package grpc
+
+import (
+	"io"
+)
+
+// Conn returns underlying connection.
+//
+// Conn is NPE-safe: returns nil if Client is nil.
+//
+// Client should not be used after Close() call
+// on the connection: behavior is undefined.
+func (c *Client) Conn() io.Closer {
+	if c != nil {
+		return c.con
+	}
+
+	return nil
+}