fix: sync the pipeline on commit()

fix: sync the pipeline on commit()

This maintains the expectation that, after commit(), the operation is
really persisted. As per postgres pipeline docs:

    The client must not assume that work is committed when it sends a
    COMMIT — only when the corresponding result is received to confirm the
    commit is complete.

With this change we are effectively receiving the result of the commit,
eventually throwing an exception if it happened.

Close #296