Turn tail recursion into iteration in CommitTransactionCommand()

Turn tail recursion into iteration in CommitTransactionCommand()

Usually the compiler will optimize away the tail recursion anyway, but
if it doesn't, you can drive the function into stack overflow. For
example:

    (n=1000000; printf "BEGIN;"; for ((i=1;i<=$n;i++)); do printf "SAVEPOINT s$i;"; done; printf "ERROR; COMMIT;") | psql >/dev/null

In order to get better readability and less changes to the existing code the
recursion-replacing loop is implemented as a wrapper function.

Report by Egor Chindyaskin and Alexander Lakhin.
Discussion: https://postgr.es/m/1672760457.940462079%40f306.i.mail.ru
Author: Alexander Korotkov, Heikki Linnakangas