Re: Revise the Asserts added to bimapset manipulation functions

I think the argument here is whether the REALLOCATE_BITMAPSETS option is
intended to force a reallocation for every modification of a bitmapset,
or only for modifications that could potentially require the set to be
reallocated.

IIUC, the v2 patch addresses the latter scenario.  I agree that it can
help find bugs in cases where there's more than one reference to a set,
and a modification that could reallocate the bitmapset might leave the
other references being dangling pointers.

It seems to me that the former scenario also makes sense in some cases.
For instance, let's say there are two pointers in two structs, s1->p and
s2->p, both referencing the same bitmapset.  If we modify the bitmapset
via s1->p (even if no reallocation could occur), s2 would see different
content via its pointer 'p'.  Sometimes this is just wrong and could
cause problems.  If we can force a reallocation for every modification
of the bitmapset, it'd be much easier to find such bugs.

Having said that, I think the codes checked in by 71a3e8c43b and
7d58f2342b are far from perfect.  And I agree that the bms_copy_and_free
in v2 patch is a good idea, as well as the bms_is_valid_set.

Thanks
Richard