Deadlock between Watcher and Debounce threads (fix #187)

There are two threads involved in `Watcher.cc` and `Debounce.cc` each
calling into respective methods of the other and thus each potentially
holding onto locks of the other. This can lead to a deadlock in the
following scenario:

While the `Debounce.cc` thread is processing callbacks in the
`Debounce::notify()` method, it holds its own lock. The method loops
over callbacks to process in `Watcher.cc` which itself requires a lock
in `Watcher.cc`. If an event gets reported while the debouncer is in
`Watcher.triggerCallbacks()`, a deadlock is present, because:
- assume the event thread is thread A
- assume the debouncer thread is thread B
- A holds its own lock in `Watcher::notify()` and calls into
  `Debounce.trigger()` which requires the debouncer lock
- B at this time is inside `Debounce::notify()` holding its own lock
  processing callbacks and about to call into `Watcher.triggerCallbacks()`
- A deadlocks waiting for B to release the debouncer lock
- B deadlocks waiting for A to release the watcher lock

src/Watcher.cc

@@ -66,6 +66,11 @@ void Watcher::notify() {
   mCond.notify_all();
 
   if (mCallbacks.size() > 0 && mEvents.size() > 0) {
+    // We must release our lock before calling into the debouncer
+    // to avoid a deadlock: the debouncer thread itself will require 
+    // our lock from its thread when calling into `triggerCallbacks` 
+    // while holding its own debouncer lock.
+    lk.unlock();
     mDebounce->trigger();
   }
 }