perf: avoid unnecessary recvfrom() in cursor.stream()

perf: avoid unnecessary recvfrom() in cursor.stream()

Call PQisBusy() before PQconsumeInput() on fetching results. If not
busy, don't call PQconsumeInput() at all but just go to fetching results
and notifications.

This is especially useful in single-row mode because most of the times
the libpq can produce several results after a single network fetch.
Previously we were calling PQconsumeInput() also when results were
already on the client and there was nothing new to read, which forced
the libpq to run a select() to tell apart a lack of data from an EOF,
see `the grumble`_, and caused the overhead reported in #286.

Close #286.

.. _the grumble: https://github.com/postgres/postgres/blob/ed57cac84d1c5642737dab1e4c4b8cb4f0c4305f/src/interfaces/libpq/fe-misc.c#L681