Updated channels to reuse logic

[0xMimir]

Added abstraction over channels service (P2pChannelsService), main reason is to simplify channels layer and reduce number of actions that perform same thing, before this each channel had its effectful counterpart which would interact with service, now each channel interacts with new actions that are abstraction over methods on P2pChannelsService.

Mental model of channels shouldn't change, only chain of actions from:

[Channel]Action -> [Channel]EffectfulAction -> Service call

to:

[Channel]Action -> Service call action -> Service call

Every channel still has its own ID and messages

Changes:

• Removed effectful action that send response/request, instead of sending message on channel like following:

dispatcher.push(P2pChannelsBestTipEffectfulAction::RequestSend { ... });
// or
dispatcher.push(P2pChannelsBestTipEffectfulAction::ResponseSend { ... });

Request/Response would be sent like following:

dispatcher.push(P2pChannelsEffectfulAction::RequestSend {
    msg: ChannelMsg::BestTipPropagation(BestTipPropagationChannelMsg::RequestMessage),
    ...
});
// or
dispatcher.push(P2pChannelsEffectfulAction::RequestSend {
    msg: ChannelMsg::BestTipPropagation(BestTipPropagationChannelMsg::ResponseMessage),
    ...
});

• Moved actions to initialize channels to single action with a callback, so instead of every channel having its own initialization action, channel id and callback should be passed to P2pChannelsEffectfulAction::InitChannel action.

Benefits:

• Easier to follow actions, only one action to open channel and one to send message, instead of every channel having its own effectful action to open channel, and instead of every message having its own action there is only one.
• Less code and less actions

[tizoc]

@0xMimir when you are done please add a description on the summary of this PR explaining the change. Skimming through it I am still not sure I understand the benefits, and it seems to be changing the mental model of how channels work (we used to have one channel per kind of message, and each channel had one specific message, now it is one channel per peer with multiple types of messages?). It is important that the difference is clear so that we can properly evaluate it.

[tizoc]

when you are done please add a description on the summary
or maybe even before depending on how much work is left (seems to be close to ready but I may be wrong).

[dkuehr]

Could you provide a detailed list of the benefits of the channel reuse logic?

I'm not fully convinced about the extra level of wrapping that is required for pushing actions.

[tizoc]

@binier if you have time, it would be useful to get your feedback on this one

[binier]

Looks good. Needs just some minor cosmetic changes.

[tizoc]

left a few more comments, but overall it LGTM now + it reduces the service interaction surface which is a big plus