Use correct conversion specifier when printing 64-bit integers

When the poll statistics are printed, the printf format specifier "%d" is used, even though the counters are 64-bit.

Instead of

dcb_printf(..., "...%d\n", ...);

the following should be used

dcb_printf(..., "...%" PRId64 "\n", ...);

The use of %d leads to wrong output when MaxScale has been running for a long period.