Clean up compilation warnings and errors

[elliottwilliams]

I was building wamped in an environment where -Werror was set, and was affected by a few simple compiler warnings:

• unused-parameter error when logging is disabled
  • I removed the intermediate variables max_len_send and serializer_type so that the expressions only evaluate when logging is enabled
  • I changed the definition of Wamp::loggedSend to not use its parameter when logging is disabled
• redirected header error on #include <sys/errno.h>, it should just be #include <errno.h>

This change fixes these issues and allows for stricter compilation :)