oauth: Fix postcondition for set_timer on macOS

oauth: Fix postcondition for set_timer on macOS

On macOS, readding an EVFILT_TIMER to a kqueue does not appear to clear
out previously queued timer events, so checks for timer expiration do
not work correctly during token retrieval. Switching to IPv4-only
communication exposes the problem, because libcurl is no longer clearing
out other timeouts related to Happy Eyeballs dual-stack handling.

Fully remove and re-register the kqueue timer events during each call to
set_timer(), to clear out any stale expirations.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://postgr.es/m/CAOYmi%2Bn4EDOOUL27_OqYT2-F2rS6S%2B3mK-ppWb2Ec92UEoUbYA%40mail.gmail.com