Add schedule.if_absent for clients that re-register periodic tasks.

schedule restarts a re-registered task's countdown, if_absent keeps the existing entry.

test/rpc/test_command_scheduler.h

@@ -0,0 +1,17 @@
+#include "test/helpers/test_main_thread.h"
+
+class TestCommandScheduler : public TestFixtureWithMainThread {
+  CPPUNIT_TEST_SUITE(TestCommandScheduler);
+
+  CPPUNIT_TEST(test_parse_rearms_existing_key);
+  CPPUNIT_TEST(test_find_locates_a_scheduled_key);
+
+  CPPUNIT_TEST_SUITE_END();
+
+public:
+  void setUp();
+  void tearDown();
+
+  void test_parse_rearms_existing_key();
+  void test_find_locates_a_scheduled_key();
+};