messenger: stop using deprecated mem_fun

When building against LLVM 9's libc++, we get a warning about using a
deprecated function. Seems like mem_fn is the appropriate replacement.

../../src/kudu/rpc/messenger.cc:238:48: warning: 'mem_fun<void, kudu::rpc::Messenger>' is deprecated [-Wdeprecated-declarations]
  *msgr = shared_ptr<Messenger>(new_msgr, std::mem_fun(&Messenger::AllExternalReferencesDropped));
                                               ^
../../thirdparty/installed/tsan/include/c++/v1/functional:1148:1: note: 'mem_fun<void, kudu::rpc::Messenger>' has been explicitly marked deprecated here
_LIBCPP_DEPRECATED_IN_CXX11 inline _LIBCPP_INLINE_VISIBILITY
^
../../thirdparty/installed/tsan/include/c++/v1/__config:944:39: note: expanded from macro '_LIBCPP_DEPRECATED_IN_CXX11'
                                      ^
../../thirdparty/installed/tsan/include/c++/v1/__config:933:48: note: expanded from macro '_LIBCPP_DEPRECATED'
                                               ^
1 warning generated.

Change-Id: Ife57c52050ba19a07218694c84989fa16f1dd341
Reviewed-on: http://gerrit.cloudera.org:8080/14834
Reviewed-by: Alexey Serbin <aserbin@cloudera.com>
Reviewed-by: Grant Henke <granthenke@apache.org>
Tested-by: Adar Dembo <adar@cloudera.com>
2 files changed