Timeline: change spec of watch_unacked_counts(). The callback may receive $error as the third argument.
t/Timeline.t: add tests for watcher quota.
Timeline: add hidden option 'watcher_max'. Implement watcher_quota.
t/Timeline.t: timeline state is restored from storage when timeline is created.
Test..StatusStorage: add test_storage_put_requires_ids() function. StatusStorage::Memory passes this test.
Timeline: update POD.