Remove 'using namespace std' statement. Fixes #276

[cguimaraes]

Fixes #276

[Mallets]

LGTM but I'd really like a review from @milyin

[cguimaraes]

I forgot to mention that the compilation as backend for zenoh-c also passes.

[milyin]

@cguimaraes this fix still doesn't work beacuse it leaks out defines

#define atomic_store_explicit std::atomic_store_explicit
#define atomic_fetch_add_explicit std::atomic_fetch_add_explicit
#define atomic_fetch_sub_explicit std::atomic_fetch_sub_explicit
#define memory_order_acquire std::memory_order_acquire
#define memory_order_release std::memory_order_release
#define memory_order_relaxed std::memory_order_relaxed

so the following code fails to compile with zenoh-pico:

    std::memory_order order = std::memory_order_relaxed;

with error:

[build] /home/milyin/ZS/zenoh-cpp/examples/universal/z_get.cxx:28:31: error: no member named 'std' in namespace 'std'; did you mean simply 'std'?
[build]     std::memory_order order = std::memory_order_relaxed;

[milyin]

the defines with names same as functions of std library still leaks to global namespace

[milyin]

Looks good now, thank you!

[cguimaraes]

@Mallets @p-avital this is ready to merge. Can you take care of it please?