merge VS solutions file from upstream master

include/grpc++/client_context.h

include/grpc++/completion_queue.h

include/grpc++/credentials.h

include/grpc++/impl/grpc_library.h

include/grpc++/server.h

include/grpc++/server_context.h

include/grpc++/stream.h

include/grpc++/time.h

src/core/support/histogram.c

src/cpp/client/channel.cc

src/cpp/client/channel.h

src/cpp/client/client_context.cc

src/cpp/client/secure_credentials.cc

src/cpp/common/completion_queue.cc

src/cpp/server/server.cc

src/cpp/server/server_context.cc

src/cpp/util/time.cc

vsprojects/grpc++/grpc++.vcxproj

vsprojects/grpc++/grpc++.vcxproj.filters

vsprojects/grpc.sln