[MINOR] switch all stat counters to 64-bit

[MINOR] switch all stat counters to 64-bit

The byte counters have long been 64-bit to avoid overflows. But with
several sites nowadays, we see session counters wrap around every 10-days
or so. So it was the moment to switch counters to 64-bit, including
error and warning counters which can theorically rise as fast as session
counters even if in practice there is very low risk.

The performance impact should not be noticeable since those counters are
only updated once per session. The stats output have been carefully checked
for proper types on both 32- and 64-bit platforms.