Delete timers before starting new ones

[ilyacodes]

In many cases, we start a new timeout with add_timeout without checking whether another timeout is already running. For timers defined by 802.11s (represented by candidate t0, t1, and t2 timers), this is not the correct behaviour – they expect a single timer to be running at a time.

Remove any existing timers with rem_timeout prior to add_timeouting a new timer setting.

Summary of behaviours:

1. Previous timer id exists, timer running —> remove old one, create new one
2. Previous timer id exists, timer expired —>
   1. case should not happen in authsae, timer id is deleted when timer expires
   2. other code using libsae MUST no-op removing an expired/non-existent timer
3. Previous timer id = 0 —> timer id 0 is reserved by libsae, removing it is no-op per 2.ii above

[bcopeland]

Looks ok... I think if we transition to estab we should also kill any outstanding timers, but plink timer in ESTAB state is a no op so it doesn't actually matter. Would be nice to wrap delete+add in a helper function.