Pluggable vault backends

[stelcheck]

Enable developers to import their own vault backend modules. This will be the first step towards externalising many of the backend modules currently in core.

[Nokitoo]

If we make the developers import their own vault backend modules the same way we are doing with our modules I think it won't be convenient. For each vault module they will need at least 3 files: index.js (class definition), Archive.js, and defaultTopicApi.js. It doesn't seem that defaultTopicApi and Archive are used elsewhere than in the vault itself. I think they could just define a get/set/list method in the vault file, and choose how to separate their code.