Problem: reset_timer() was being called without checking for whether the
client state for the client_id still existed. debounce_request() starts
a timer that defers a call to send_request() which then calls
reset_timer(). If the timer fires after the client_state is erased, then
the deferred function attempts to dereference the timer on a nil client
state.
Solution: Change reset_timer to take a state directly so it can't be nil
and move the reset_timer() call inside a guard that ensures state
exists. Additionally, reset a client's timer when the client detaches so
it doesn't become dangling.
f0559997dd