Define rough architecture for middleware

[TheCharlatan]

The functionality of the middleware is now split into three packages. The middleware package takes care of communicating with bitcoind and lightningd and emits an event if new data is received.

The handlers package takes care of the communication with the bitbox-wallet-app. It upgrades connections to websocket, if requested, and starts the main middleware event loop. Events from the
middleware are caught and passed into a websocket handler.

The main package is what is compiled into a binary. It parses command line arguments and creates new middleware and handler instances.

This commit also implements a coherent logging structure, which is handled in the same fashion as in the bitbox-wallet-app.

[hkjn]

Thanks for the PR!

See some comments inline from an initial review.

I noticed this is a large change (+231k LoC); see comment inline discussing some alternatives.

It also wasn't 100% clear to me if this change was entirely a refactor, or if there's also new features added (beyond logging).

Would it be possible to split out more granular commits, to simplify the review process and merge the easier parts first?

[hkjn]

I discussed this PR with @TheCharlatan offline, and the CI that was added in #34 should be passing at 80f9a76.

Even with using the github.com/sirupsen/logrus package directly, we still end up pulling in ~230k LoC, so we'll instead plan to not use any fancy logging package and leave some guidelines for how to structure log output for future developers.

[hkjn]

We looked briefly into why github.com/sirupsen/logrus actually needs ~230k LoC, and it seems that ~225k LoC was due to the package depending on golang.org/x/sys/unix, which seems to just be used to detect if the caller is on UNIX, MacOs or Windows. There seems to have been some prior efforts (e.g sirupsen/logrus#930) to reduce the size of the logrus dependency graph, so for now we should probably stick to the recommendation of not using that package. If we can manage without any 3rd party package at all for logging (i.e just using fmt and log packages) that'd be easiest.

[hkjn]

Thanks for the changes to no longer depend on logrus, this is now a much smaller patch than before!

I've left some more comments inline.

[hkjn]

Thanks, see few more comments inline.

[hkjn]

Thanks for the changes!

ACK 9d4545a