Revert to 9.6 treatment of ALTER TYPE enumtype ADD VALUE.
This reverts commit
15bc038f9, along with the followon commits
1635e80d3
and
984c92074 that tried to clean up the problems exposed by bug #14825.
The result was incomplete because it failed to address parallel-query
requirements. With 10.0 release so close upon us, now does not seem like
the time to be adding more code to fix that. I hope we can un-revert this
code and add the missing parallel query support during the v11 cycle.
Back-patch to v10.
Discussion: https://postgr.es/m/
20170922185904.1448.16585@wrigleys.postgresql.org