gc: use wide integer for schedule counting

Currently we use unsined int for "cleanup" schedule
counting, this is safe while this routine is not
called too often. Still there is a chance to hit
a number wrap on code modification because there
is no strict rule on how to use this garbage collector.
Lets use wide integers instead, we have only one gc
instance and such approach eliminates potential problems
in future (actually this should had been done from the
beginning since the current gc code flow developed
without wrapping in mind).

In-scope-of #5806

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

src/box/gc.c

@@ -239,7 +239,7 @@ gc_cleanup_fiber_f(va_list ap)
 {
 	(void)ap;
 	while (!fiber_is_cancelled()) {
-		int delta = gc.cleanup_scheduled - gc.cleanup_completed;
+		int64_t delta = gc.cleanup_scheduled - gc.cleanup_completed;
 		if (delta == 0) {
 			/* No pending garbage collection. */
 			fiber_sleep(TIMEOUT_INFINITY);
@@ -278,7 +278,7 @@ gc_schedule_cleanup(void)
 static void
 gc_wait_cleanup(void)
 {
-	unsigned scheduled = gc.cleanup_scheduled;
+	int64_t scheduled = gc.cleanup_scheduled;
 	while (gc.cleanup_completed < scheduled)
 		fiber_cond_wait(&gc.cleanup_cond);
 }

src/box/gc.h

@@ -146,7 +146,7 @@ struct gc_state {
 	 * sleep until @completed reaches the value of @scheduled
 	 * taken at that moment of time.
 	 */
-	unsigned cleanup_completed, cleanup_scheduled;
+	int64_t cleanup_completed, cleanup_scheduled;
 	/**
 	 * Set if there's a fiber making a checkpoint right now.
 	 */