Implement the charm store logging API.

This will allow storing/retrieving ingestion logs.
To activate the functionality, a client branch is also
required.
This is pretty unusual, and I'm not sure that JS clients will be able to read a sequence of JSON messages like this.

However I like the idea of streaming the messages to avoid potentially using up loads of memory.

One alternative could be to manually write the first "[", subsequent comma separators, and the final "]".

That way we can still stream everything, but the client will see the whole thing as a single JSON array.