Clean up misuse and nonuse of poll_query_until().

Tom Lane <tgl@sss.pgh.pa.us>

Commit: b0f069d931f0a3d4a39aeeb230baf2f2b18cb3c3
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-07-01T18:25:09Z
Releases: 10.0
Clean up misuse and nonuse of poll_query_until().

Several callers of PostgresNode::poll_query_until() neglected to check
for failure; I do not think that's optional.  Also, rewrite one place
that had reinvented poll_query_until() for no very good reason.

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