libhdfs++ Fix valgrind failures when using more than 1 io_service worker thread.

Valgrind catches an invalid read of size 8. Setup: 4 io_service worker threads, 64 threads doing open-read-close on a small file.

Stack:
==8351== Invalid read of size 8
==8351== at 0x51F45C: asio::detail::reactive_socket_recv_op<asio::mutable_buffers_1, asio::detail::read_op<asio::basic_stream_socket<asio::ip::tcp, asio::stream_socket_service<asio::ip::tcp> >, asio::mutable_buffers_1, asio::detail::transfer_all_t, std::_Bind<std::_Mem_fn<void (hdfs::RpcConnectionImpl<asio::basic_stream_socket<asio::ip::tcp, asio::stream_socket_service<asio::ip::tcp> > >::)(std::error_code const&, unsigned long)> (hdfs::RpcConnectionImpl<asio::basic_stream_socket<asio::ip::tcp, asio::stream_socket_service<asio::ip::tcp> > >, std::_Placeholder<1>, std::_Placeholder<2>)> > >::do_complete(asio::detail::task_io_service*, asio::detail::task_io_service_operation*, std::error_code const&, unsigned long) (functional:601)
==8351== by 0x508B10: hdfs::IoServiceImpl::Run() (task_io_service_operation.hpp:37)
==8351== by 0x55BCBEF: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.19)
==8351== by 0x5A2D181: start_thread (pthread_create.c:312)
==8351== by 0x5D3D47C: clone (clone.S:111)
==8351== Address 0x67e3eb0 is 0 bytes inside a block of size 216 free'd
==8351== at 0x4C2C2BC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==8351== by 0x51F7B2: hdfs::RpcConnectionImpl<asio::basic_stream_socket<asio::ip::tcp, asio::stream_socket_service<asio::ip::tcp> > >::~RpcConnectionImpl() (rpc_connection.h:32)
==8351== by 0x50C104: hdfs::FileSystemImpl::~FileSystemImpl() (unique_ptr.h:67)
==8351== by 0x503A10: hdfs::HadoopFileSystem::~HadoopFileSystem() (unique_ptr.h:67)
==8351== by 0x503B28: hdfs::HadoopFileSystem::~HadoopFileSystem() (hdfs_cpp.cc:140)
==8351== by 0x503580: hdfs_internal::~hdfs_internal() (unique_ptr.h:67)
==8351== by 0x502FEE: hdfsDisconnect (hdfs.cc:127)
==8351== by 0x5010B7: main (threaded_stress_test.cc:74)
==8351==
pure virtual method called
terminate called without an active exception