dev/watchman: merge file change events while running handle changed

Created by: keegancsmith

Watchman only sends file change events once the FS is settled. However, it isn't aware of our handleChanged process, so if a dev is still saving files while the compiler is running the events would queue up. This would lead to our watchman wrapper running handle changed several times. We now dedup the watch events while the compilation is running before calling it again.