Pass buffer size as `size_t`.
Pass buffer size as `size_t`.

Fixes #266.

I had wanted to do this, but forgot.  It's a shame that the same
function now has the size of the buffer as two different types, but
`PQcancel` takes an `int` whereas `std::string` takes a `size_t`
(more or less).

jtv/libpqxx,jtv/libpqxx,jtv/libpqxx,jtv/libpqxx