Commit f0e44751 added new node tags at a place in the tag numbering
where there was no daylight left before the next hard-coded number,
resulting in some duplicate tag assignments.  This doesn't seem to have
caused any big problem so far, but it's surely trouble waiting to happen.

We could adjust the manually assigned breakpoints to make more room,
but that just leaves the same hazard waiting to strike again in future.
What seems like a better idea is to get rid of the manual assignments
and leave NodeTags to be automatically assigned, consecutively from one
on up.  This means that any change in the tag list forces a backend-wide
recompile, but realistically that's usually needed anyway.

Discussion: https://postgr.es/m/29670.1482942811@sss.pgh.pa.us