[WIP] Prototype for an action-based Store API

As another attempt to design a Store API that would allow implementors to make proper optimizations on store access operations, I tried to make one that is based on actions rather than generic CRUD operations. Now I fully understand the need to have something action-specific rather than trying to be too generic, I hope we can come to a viable solution this time.

For that there were basically two options, either have a huge interface with dozens of methods for every possible action on the store, or have a common interface for individual actions, reducing the Store interface to just one method that accepts an action as parameter. I went for the second option, because it would allow more things like making user-defined actions, have Stores that only handle a subset of actions, and most importantly not break backward-compatibility when adding a new action to the API.

Here are a few things to understand about the content of this PR:

I don't know if you want to put the old API in a deprecation state before removing it completely, so I kept the old API and made the new API in the discord4j.store.api.wip package for the time being
This PR is a draft as there are missing classes and javadocs. I will wait for the team to validate the API structure first.
SwitchingStore is basically the equivalent of the current MappingStoreService. Except that is is capable of filtering on actions instead of entities, and it can also multi-cast an action onto several stores.
The generic type <R> on StoreAction corresponds to the expected return type of the action. It is only used to infer the return type of Store#execute.
The class StoreActionHandlingHelper exists because the implementations of Store would otherwise have to use tons of instanceof and also make tons of unsafe casts because of the <R> generic parameter. It is not mandatory for implementors to use it, hence why I've put it in an util package, but I'm sure that could help a lot.
The hierarchy of action contains empty intermediate interfaces such as ReadAction, GatewayWriteAction, etc. They don't provide extra methods, but will be very useful to write conditions for a SwitchingStore, and also give the ability to make read-only stores and whatnot.
Read actions will be common for each entity, for now I've only made two for Channels, will add the rest later. For write actions however I made it so that on the one hand you have actions triggered by gateway events (GatewayWriteAction), and on the other hand write actions that could be performed manually (ExternalWriteAction), which can open up the path for REST caching. The Store implementation is free to support either only Gateway or External, or both.

Some example usage of the new API:

GatewayDiscordClient gateway = client.gateway()
        .setStore(SwitchingStore.builder()
                .useIfActionOfType(new JdkStore(), ReadAction.class, GatewayWriteAction.class)
                .useIfActionOfType(new NoOpStore(), ExternalWriteAction.class)
                .build())
        .login()
        .block();

How to use a store:

ChannelData data = store.execute(new GetChannelByIdAction(channelId)).block();

How to implement a store using the helper class:

public class MyCustomStore implements Store {

    @Override
    public <R> Mono<R> execute(StoreAction<R> action) {
        return StoreActionHandlingHelper.create()
                .addHandler(GuildCreateAction.class, this::handleGuildCreate)
                .addHandler(GuildDeleteAction.class, this::handleGuildDelete)
                .handle(action);
    }

    private Mono<Void> handleGuildCreate(GuildCreateAction action) {
        //...
    }

    private Mono<GuildData> handleGuildDelete(GuildDeleteAction action) {
        //... 
    }
}

Discord4J / Stores

[WIP] Prototype for an action-based Store API #9