Fix truncation of bytes/bytes_read/bytes_written stats

These stats are stored in a `uint64_t` but we were truncating them to 32 bits.
Newer version of libmemcached do [use strtoll](http://bazaar.launchpad.net/~tangent-trunk/libmemcached/1.2/view/head:/libmemcached/stats.cc#L210), so I suspect you're correct. However what I changed the code to here matches the other 64-bit stats (like `cmd_get` a few lines below the end of this diff). I wanted to make as small a change as possible to fix the immediate issue, which is truncating to 32 bits. It seems unlikely that we would end up with a value greater than 2^63 (8 exabytes) here.