pgsql: Generalize parallel slot result handling.

Generalize parallel slot result handling.

Instead of having a hard-coded behavior that we ignore missing
tables and report all other errors, let the caller decide what
to do by setting a callback.

Mark Dilger, reviewed and somewhat revised by me. The larger patch
series of which this is a part has also had review from Peter
Geoghegan, Andres Freund, Álvaro Herrera, Michael Paquier, and Amul
Sul, but I don't know whether any of them have reviewed this bit
specifically.

Discussion: http://postgr.es/m/12ED3DA8-25F0-4B68-937D-D907CFBF08E7@enterprisedb.com
Discussion: http://postgr.es/m/5F743835-3399-419C-8324-2D424237E999@enterprisedb.com
Discussion: http://postgr.es/m/70655DF3-33CE-4527-9A4D-DDEB582B6BA0@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/418611c84d004f45d92bcaa3f8e100385d96cd41

Modified Files
--------------
src/bin/scripts/reindexdb.c | 1 +
src/bin/scripts/vacuumdb.c | 1 +
src/fe_utils/parallel_slot.c | 91 +++++++++++++++++++++++++-----------
src/include/fe_utils/parallel_slot.h | 29 ++++++++++++
4 files changed, 94 insertions(+), 28 deletions(-)