pgsql: Fix (some of the) breakage introduced into query-cancel

Log Message:
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Fix (some of the) breakage introduced into query-cancel processing by HS.

It is absolutely not okay to throw an ereport(ERROR) in any random place in
the code just because DoingCommandRead is set; interrupting, say, OpenSSL
in the midst of its activities is guaranteed to result in heartache.

Instead of that, undo the original optimizations that threw away
QueryCancelPending anytime we were starting or finishing a command read, and
instead discard the cancel request within ProcessInterrupts if we find that
there is no HS reason for forcing a cancel and we are DoingCommandRead.

In passing, may I once again condemn the practice of changing the code
and not fixing the adjacent comment that you just turned into a lie?

Modified Files:
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    pgsql/src/backend/tcop:
        postgres.c (r1.580 -> r1.581)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c?r1=1.580&r2=1.581)