Move the commons and background adapter into this repository

[pepijn]

I don't like background functions because:

• It's not possible to expose a health check endpoint to see what build is running (no HTTP endpoint available to call).
• it's impossible to do an synchronous invocation using the gcloud CLI.
• They don't seem to support retries and dead lettering (even with dead letters enabled, in the case of pubsub triggered background functions).

But there might still be valid use cases for it so we should support it. The libraries are very close so it makes sense to move them into the same repository. While still having separate Maven coordinates, however.

Also, rename the repo to something like google-cloud-function-clojure-adapters.