synchronized grpc cpp_generator with latest version as of today: grpc-1.8.1.  (#4556)

* updateed cpp_generator.cc to be compatible with the latest grpc version

* preserved the original license

* synchronized grpc cpp_generator with latest version as of today: GRPC 1.8.1. Regenerated test/monster_test.grpc.fb.* files and verified that grpctest is nicely passing

* fixed merge glitch
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c3974b3..238ecdc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -244,7 +244,7 @@
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-parameter -Wno-shadow")
   endif()
   add_executable(grpctest ${FlatBuffers_GRPCTest_SRCS})
-  target_link_libraries(grpctest grpc++_unsecure grpc_unsecure pthread dl)
+  target_link_libraries(grpctest grpc++_unsecure grpc_unsecure gpr pthread dl)
 endif()
 
 if(FLATBUFFERS_INSTALL)