Eliminate copies in deferred cleanup

[TimSylvester]

Pulls in an implementation of move_only_function.

This simplifies deferred deletions, since there's no temporary copy to potentially outlast the schedule call, and eliminates the need to convert unique_ptrs to shared_ptrs.

Unfortunately, the change of function types for the scheduler spreads to the map callbacks due to usage like:

pathChangeCallback = Scheduler::GetCurrent()->bindOnce(...

This is further complicated by the fact that of the callbacks are explicitly copied:

void FileSourceRequest::setResponse(const Response& response) {
    // Copy, because calling the callback will sometimes self
    // destroy this object. We cannot move because this method
    // can be called more than once.

Also fixes various style warnings.

Working in Android and ios so far.

Is this worth pursuing further?