fix: avoid all uses of PQconn.send_query in pipeline mode

fix: avoid all uses of PQconn.send_query in pipeline mode

We stopped using it in normal querying because, until libpq 14.4, the
libpq complained about "message type 0x33 arrived from server while
idle" (see #314). But usage remained for internal queries such as BEGIN.

In libpq 14.5, Postgres devs tried to fix the issue... and broke
PQsendQuery altogether :(

Hopefully the issue will be solved in future libpq versions, but we can
work around it completely.

Close #350.