nng_closeall() is now deprecated and should not be used (generate many compilation warnings)

[Florent Teichteil-Koenigsbuch]

The nng::close_all() function calls the nng_closeall() from nng which is deprecated:

• where it is called:
  

  nngpp/include/nngpp/core.h

  Line 39 in 8da8c02

   nng_closeall(); 

• original function which is now deprecated:
  https://github.com/nanomsg/nng/blob/a6de76c0b1e08ceba5c3e47dd1107f1a09490b0d/include/nng/nng.h#L1325

As a result, compiling code that includes nngpp/core.h generates many deprecation warnings that could result in compilation errors the day when nng will completely remove the nng_closeall() function from their code base.