Thread safety

[vmaraccini]

Make reductions thread-safe and make Store subscriptions fire on the Main Thread.

This PR proposes a way to make store reductions thread-safe by using objc_sync_enter and objc_sync_exit synchronization primitives, using the Store itself as a lock.

open func _defaultDispatch(action: Action) {
    objc_sync_enter(self)
    let newState = reducer(action, state)
    state = newState
    objc_sync_exit(self)
}

TODO:

• Allow for the selection of a callback queue where newState methods will be called. The currently preferred way to do this would be to add an optional parameter when subscribing to store changes.

[luizdubas]

why did you remove the mainThread enforcement?

[vmaraccini]

This should be optional; Always calling newState in the main thread may be unnecessary, or even undesired.