arti: Make the config watcher debounce interval configurable.

Previously, the reload_cfg::test::watch_multiple test would take about 3s to run. This test modifies 3 files and waits for the corresponding reconfigure() events to fire. Because of the sleep(DEBOUNCE_INTERVAL) in run_watcher, it would wind up waiting for about 1s for each of them.

This makes the event debouncing optional, and disables it in the tests.

Closes #1589 (closed)