Avoid namespace ambiguity

[Felix-El]

Hi,

thanks for sharing the patch. I'm trying it locally.

However, I've faced issues compiling one existing node with the patched roscpp: the node defines a time_point at global scope and later, when condition_variable.h is included through a chain of other include files, we have an ambiguity there.

It sure is not good practice to have stuff at global scope but still, to avoid the issue for others, I'd propose to get rid of 'using namespace boost::chrono;' in favor of a namespace alias.

Guess I just learned that even scoped 'using namespace ...' is not great for use in header files, since the environment we're compiling in is controlled by the includer.

Best Regards

-Felix