Implement opt.stats_interval and the _opts options.

Add options stats_interval and stats_interval_opts to allow interval based stats
printing.  This provides an easy way to collect stats without code changes,
because opt.stats_print may not work (some binaries never exit).

src/prof.c

@@ -571,7 +571,10 @@ void
 prof_idump_rollback_impl(tsdn_t *tsdn, size_t usize) {
 	cassert(config_prof);
 
-	return counter_rollback(tsdn, &prof_idump_accumulated, usize);
+	/* Rollback is only done on arena_prof_promote of small sizes. */
+	assert(SC_LARGE_MINCLASS > usize);
+	return counter_rollback(tsdn, &prof_idump_accumulated,
+	    SC_LARGE_MINCLASS - usize);
 }
 
 bool