Minor refactorings to eliminate some static buffers

As part of the multithreading work, it'd be nice to get rid of as many
global or static variables as possible. Remaining ones can be converted
to thread locals as appropriate, but where possible, it's better to just
get rid of them.

Here are patches to get rid of a few static variables, by e.g.
converting them to regular local variables or palloc'd return values, as
appropriate.

This doesn't move the needle much, but every little helps, and these
seem like nice little changes in any case.

--
Heikki Linnakangas
Neon (https://neon.tech)