Use mutexes for byte counts to fix i386/arm panic.

This commit changes the server byte counters over to use a mutex instead
of the atomic package.  The atomic.AddUint64 function requires the struct
fields to be 64-bit aligned on 32-bit platforms.  The byte counts are
fields in the server struct and are not 64-bit aligned.  While it would be
possible to arrange the fields to be aligned through various means, it
would make the code too fragile for my tastes.  I prefer code that doesn't
depend on platform specific alignment.

Fixes #96.