This alist was meant to map buffers to timers, but the "push" function
was not checking for already-existing timers.  This was causing a
flood of PINGs being queued (because more than one timer was inserting
those), and the flood-control logic would block those from going
through quickly.

This patch checks for existing timers in the alist before adding new
ones.  If a timer already exists, it is cancelled and overwritten.