[TIMOB-26574] TiAPI: Implement set/clearImmediate

setImmediate and clearImmediate are *sort of* like setTimeout/clearTimeout, but are supposed to be run after current I/O event callbacks are done. https://nodejs.org/api/timers.html#timers_setimmediate_callback_args Basically this is roughly like setTimeout(callback, 0); as a first stab, but should be run **before** and timeout callbacks. In effect, immediate, timers and ticks from process.nextTick are *sort of* like 3 separate priorities of timer queues, although at a lower level they have semantic/behavioral differences. ticks are right away and can cause infinite loops. immediate come next chance after I/O events callbacks. Then timers get fired when ready.