Fix race condition in startup progress reporting.

Fix race condition in startup progress reporting.

Commit 9ce346eabf350a130bba46be3f8c50ba28506969 added startup
progress reporting, but begin_startup_progress_phase has a race
condition: the timeout for the previous phase might fire just
before we reschedule the interrupt for the next phase.

To avoid the race, disable the timeout, clear the flag, and then
re-enable the timeout.

Patch by me, reviewed by Nitin Jadhav.

Discussion: https://postgr.es/m/CA+TgmoYq38i6iAzfRLVxA6Cm+wMCf4WM8wC3o_a+X_JvWC8bJg@mail.gmail.com