Refactor RPCProtocol

[tsmaeder]

What it does

This refactoring clarifies an decouples a bunch of things. This is a pure refactoring which should not change behaviour.

1. Each connection to a plugin host has an explicit host id: we already have this concept in HostedPluginSupport#initRpc, but instead of using the already known host id, we use the id of the first plugin deployed on the plugin host for a connection id.
2. JsonPlugin(Client|Server) now explicitly reference a plugin host id instead of encoding that in the messages being transferred
3. MessageConnection only handle strings now: we're sending json messages, so this is not really a restriction.
4. RPCMultiplexer now is an implementation detail of RPCProtocol.
5. RPCProtocol only deals with sending remote invocations over a connection: all things specific to plugin hosts and routing have been removed.

How to test

Run Theia with with at least a back-end and one front-end plugin. No exceptions related to rpc processing should appear in neither the back-end nor front-end logs.
Note: front-end plugins still don't work with electron.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[paul-marechal]

I don't fully understand the system, but so far those changes are looking good to me.

[tsmaeder]

@marechal-p thx for the review.

[benoitf]

Hello, I still don't get the real benefits, (clarifies is subjective there) except maybe introducing new bugs ?

[benoitf]

I don't see regression using Theia

[paul-marechal]

@tsmaeder merge at will.

[tsmaeder]

@marechal-p Not merging because of downstream reasons 🤷 .