API formatting

[BLuedtke]

Currently, the API returns results in a pure text-based manner, with some similar formats across different handler, but without a specification for it. This makes parsing a chore.
A common formatting standard (e.g., JSON) should help make parsing easier. However, which standard to choose and how to name its fields has to be discussed still.

[BLuedtke]

First prototype for /monitor/* API implemented in linked branch (json-experiments). But clients have not been adjusted there. Also, need to check if any other APIs need changing.

[BLuedtke]

The API format rework has now evolved into a significant rewrite/reformatting of the major handler_X implementations. As I'm significantly modifying these handlers anyway, I'm fixing a few issues with them and also will improve upon some logging, locking etc. aspects. I propose then running this version in a 'dev'-like staging branch for a while, until the clients are also adapted and tested.