job: Refactor to use pointers instead of ids

'job_start' returns the id as an out paramter, and the 'job_find' function is
now used by eval.c to translate job ids into pointers.
job: Add defer flag and setter method

This is has the same effect as the RStream 'defer' flag, but also works for the
job's exit event.
events: Refactor event_poll to use stack-allocated timer handles
events: Refactor how events are queued for processing

To make it possible reuse `event_poll` recursively and in other blocking
function calls, this changes how deferred/immediate events are processed:

- There are two queues in event.c, one for immediate events and another for
  deferred events. The queue used when pushing/processing events is determined
  with boolean arguments passed to `event_push`/`event_process` respectively.
- Events pushed to the immediate queue are processed inside `event_poll` but
  after the `uv_run` call. This is required because libuv event loop does not
  support recursion, and processing events may result in other `event_poll`
  calls.
- Events pushed to the deferred queue are processed later by calling
  `event_process(true)`. This is required to "trick" vim into treating all
  asynchronous events as special keypresses, which is the least obtrusive
  way of introducing asynchronicity into the editor.
- RStream instances will now forward the `defer` flag to the `event_push` call.