refactor(time): refactor delay with input checking

Previously, there were three low-level delay entry points

- os_delay(ms, ignoreinput=true): sleep for ms, only break on got_int

- os_delay(ms, ignoreinput=false): sleep for ms, break on any key input
  os_microdelay(us, false): equivalent, but in μs (not directly called)

- os_microdelay(us, true): sleep for μs, never break.

The implementation of the latter two both used uv_cond_timedwait()
This could have been for two reasons:
 1. allow another thread to "interrupt" the wait
 2. uv_cond_timedwait() has higher resolution than uv_sleep()

However we (1) never used the first, even when TUI was a thread, and
(2) nowhere in the codebase are we using μs resolution, it is always a ms
multiplied with 1000.

In addition, os_delay(ms, false) would completely block the thread for
100ms intervals and in between check for input. This is not how event handling
is done alound here.

Therefore:

Replace the implementation of os_delay(ms, false) to use
LOOP_PROCESS_EVENTS_UNTIL which does a proper epoll wait with a timeout,
instead of the 100ms timer panic.

Replace os_microdelay(us, false) with a direct wrapper of uv_sleep.

src/nvim/main.c

@@ -173,7 +173,7 @@ bool event_teardown(void)
 
 /// Performs early initialization.
 ///
-/// Needed for unit tests. Must be called after `time_init()`.
+/// Needed for unit tests.
 void early_init(mparm_T *paramp)
 {
   estack_init();
@@ -261,7 +261,6 @@ int main(int argc, char **argv)
   mparm_T params;         // various parameters passed between
                           // main() and other functions.
   char *cwd = NULL;       // current working dir on startup
-  time_init();
 
   // Many variables are in `params` so that we can pass them around easily.
   // `argc` and `argv` are also copied, so that they can be changed.