improve connection timer logic (#5339)

improve connection timer logic (#5339)

* simplify timer resets

As of Go 1.23, timer resets work as expected, and there’s no need to
read a (potentially) stale value from the timer channel.

* don't save the last time

* track if blocked

* remove unblock after sendingScheduled

* fix unblock logic when sending

* don't use fallthrough

* track the blocking mode

* remove stale comment