BUG/MINOR: spoa-server: fix size_t format printing

BUG/MINOR: spoa-server: fix size_t format printing

From https://www.python.org/dev/peps/pep-0353/
"A new type Py_ssize_t is introduced, which has the same size as the
compiler's size_t type, but is signed. It will be a typedef for ssize_t
where available."

For integer types, causes printf to expect a size_t-sized integer
argument.

This should fix github issue #702

This should be backported to >= v2.2

Signed-off-by: William Dauchy <w.dauchy@criteo.com>