Merge: github: Summer cleaning

Some cleaning to the `github` lib.

Most interesting changes:

* kill `github_curl` and migrate clients (`github_merge`, `github_search_for_jni`)
* clean deserialization process heuristics for something more robust and readable (but a bit slower)
* array pagination
* kill `GithubEntity`
* remove ISO date methods
* mock testing for `github::wallet`
* `wallet` uses a `Logger`

This PR can be read commit by commit.

Pull-Request: #2765
Reviewed-by: Jean Privat <jean@pryen.org>