Revisit anonymous namespaces vs. static functions

[clalancette]

C++ has two different ways to create compilation-unit-local functions: using an anonymous namespace (the modern C++ way), or by putting static in front of the function (the C way).

Currently in rmw_zenoh_cpp we use a mixture of these two styles. We should be consistent here, and just choose one. My personal preference is for static in front of the function, as it is easier to tell at a glance that a function is local-only. But I'm open to other opinions.

This comes out of a discussion in #123 (comment)

[fujitatomoya]

@clalancette can you assign this to @Yadunund ?