Remove Coroutines from public API & introduce Coroutines and Reactive modules

[alexstyl]

This PR removes any Coroutine suspend or Flows from the public API of ContactStore.

Instead of the Flow, the API exposes a new FetchRequest object, which allows the consumers to either get the result in a block manner throughblockingGet() or collect() in an asynchronous fashion.

Example of usage:

val fetchJob = contactStore.fetchContacts()
    .collect {
        state.value = Loaded(it)
    }

// onDestroy()
fetchJob?.cancel()

similarly, the fetchContactGroup() function returns a FetchRequest which emits a list of ContactGroup.

The PR also intoduces 2 new modules: coroutines and reactive. The modules contain extension functions for those that are familar working with coroutines and rx-java3 respectively.

Example of usage using the coroutines extensions:

val job = contactStore.fetchContacts().asFlow()
    .collect {
        state.value = Loaded(it)
    }

// onDestroy()
job?.cancel()

example of usage using reactive extensions:

val disposable = contactStore.fetchContacts().asFlowable()
    .subscribe {
        state.value = Loaded(it)
    }

// onDestroy()
disposable?.cancel()