events: Refactor how event deferral is handled

- Remove all *_set_defer methods and the 'defer' flag from rstream/jobs
- Added {signal,rstream,job}_event_source functions. Each return a pointer that
  represent the event source for the object in question(For signals, a static
  pointer is returned)
- Added a 'source' field to the Event struct, which is set to the appropriate
  value by the code that created the event.
- Added a 'sources' parameter to `event_poll`. It should point to a
  NULL-terminated array of event sources that will be used to decide which
  events should be processed immediately
- Added a 'source_override' parameter to `rstream_new`. This was required to use
  jobs as event sources of RStream instances(When "focusing" on a job, for
  example).
- Extracted `process_from` static function from `event_process`.
- Remove 'defer' parameter from `event_process`, which now operates only on
  deferred events.
- Refactor `channel_send_call` to use the new lock mechanism

What changed in a single sentence: Code that calls `event_poll` have to specify
which event sources should NOT be deferred. This change was necessary for a
number of reasons:

- To fix a bug where due to race conditions, a client request
  could end in the deferred queue in the middle of a `channel_send_call`
  invocation, resulting in a deadlock since the client process would never
  receive a response, and channel_send_call would never return because
  the client would still be waiting for the response.
- To handle "event locking" correctly in recursive `channel_send_call`
  invocations when the frames are waiting for responses from different
  clients. Not much of an issue now since there's only a python client, but
  could break things later.
- To simplify the process of implementing synchronous functions that depend on
  asynchronous events.

src/nvim/os/signal.c

@@ -103,6 +103,11 @@ void signal_handle(Event event)
   }
 }
 
+EventSource signal_event_source(void)
+{
+  return &sint;
+}
+
 static char * signal_name(int signum)
 {
   switch (signum) {
@@ -155,10 +160,11 @@ static void signal_cb(uv_signal_t *handle, int signum)
   }
 
   Event event = {
+    .source = signal_event_source(),
     .type = kEventSignal,
     .data = {
       .signum = signum
     }
   };
-  event_push(event, true);
+  event_push(event);
 }