Fix volatility marking of commit timestamp functions

They are marked stable, but since they act on instantaneous state and it
is possible to consult state of transactions as they commit, the results
could change mid-query.  They need to be marked volatile, and this
commit does so.

There would normally be a catversion bump here, but this is so much a
niche feature and I don't believe there's real damage from the incorrect
marking, that I refrained.

Backpatch to 9.5, where commit timestamps where introduced.

Per note from Fujii Masao.