Only read one record at a time in sptps_receive_data().

Only read one record at a time in sptps_receive_data().

sptps_receive_data() always consumes the entire buffer passed to it,
which is somewhat inflexible. This commit improves the interface so that
sptps_receive_data() consumes at most one record. The goal is to allow
non-SPTPS stuff to be interleaved with SPTPS records in a single TCP
stream.