core: introduce serial_timer_task

This is a wrapper of timer_task which should be used if atomicity of
callback tasks and timer operations is required. The class accepts
external lock to serialize all operations. It provides sufficient
abstraction to replace callouts in the network stack.

Unfortunately, it requires some cooperation from the callback code
(see try_fire()). That's because I couldn't extract in_pcb lock
acquisition out of the callback code in TCP stack because there are
other locks taken before it and doing so _could_ result in lock order
inversion problems and hence deadlocks. If we can prove these to be
safe then the API could be simplified.

It may be also worthwhile to propagate the lock passed to
serial_timer_task down to timer_task to save extra CAS.