Provide a generic client-side unary callback API

src/cpp/server/server_context.cc

diff --git a/src/cpp/server/server_context.cc b/src/cpp/server/server_context.cc
index 6f5bde0..f76f64e 100644
--- a/src/cpp/server/server_context.cc
+++ b/src/cpp/server/server_context.cc
@@ -61,6 +61,9 @@
     tag_ = tag;
   }
 
+  /// TODO(vjpai): Allow override of cq_tag if appropriate for callback API
+  void* cq_tag() override { return this; }
+
   void Unref();
 
  private: