fix: fix Transaction commit in pipeline mode

fix: fix Transaction commit in pipeline mode

Make sure we receive the commit message, so we can raise an immediate
exception, as we do for connection-handled transactions.

Highlight, in the test, a behaviour difference between commit in
pipeline and non-pipeline mode. In the latter, after a failed commit,
the connection is left IDLE; in pipeline mode it is left INERROR so we
need a further rollback to keep on using it. Will look if this can be
made consistent.