Wake up background threads on demand

This change allows every allocator conforming to PAI communicate that it
deferred some work for the future. Without it if a background thread goes into
indefinite sleep, there is no way to notify it about upcoming deferred work.

test/unit/decay.c

@@ -49,7 +49,7 @@ TEST_BEGIN(test_decay_npages_purge_in) {
 	expect_false(decay_init(&decay, &curtime, (ssize_t)decay_ms),
 	    "Failed to initialize decay");
 
-	const size_t new_pages = 100;
+	size_t new_pages = 100;
 
 	nstime_t time;
 	nstime_copy(&time, &decay_nstime);