buf_page_ibuf_merge_try(): A new, separate function for invoking
ibuf_merge_or_delete_for_page() when needed. Use the already requested
page latch for determining if the call is necessary. If it is and
if we are currently holding rw_latch==RW_S_LATCH, upgrading to an exclusive
latch may involve waiting that another thread acquires and releases
a U or X latch on the page. If we have to wait, we must recheck if the
call to ibuf_merge_or_delete_for_page() is still needed. If the page
turns out to be corrupted, we will release and fail the operation.
Finally, the exclusive page latch will be downgraded to the originally
requested latch.

ssux_lock_impl::rd_u_upgrade_try(): Attempt to upgrade a shared lock to
an update lock.

sux_lock::s_x_upgrade_try(): Attempt to upgrade a shared lock to
exclusive.

sux_lock::s_x_upgrade(): Upgrade a shared lock to exclusive.
Return whether a wait was elided.

ssux_lock_impl::u_rd_downgrade(), sux_lock::u_s_downgrade():
Downgrade an update lock to shared.