Do not try to be too smart which compilations need config.h, as most of
them will simply because of the config.h has information about system
capabilities.
* Calling Timer::setTimeout() from within Timer::start() might lead to ugly
behavior (as experienced in bugs #3590078, #3600143, etc; see commit
4d307dcd10, fixes the problem partially).
* Stop a timer first, then call the handler (via Timer::fireTimeout()). A
given handler might call Timer::start() again, which (re)adds the Timer
to the control list .. the following Timer::stop() would remove it again.
* Use 'm_start' as indicator if timer is running.
* Move the (now quite short) code of ::addTimer / ::removeTimer
into the Timer::start() and Timer::stop() functions.
gettimeofday() is subject to be changed on daylight-saving or to ntp-related
(think leap-seconds). even worse, it is subject to be changed BACK in time. this
is hard to fix correctly (see commit 45726d3016 and bug #3560509). it is
irrelevant for timers to know the nano-seconds since the epoch anyways.