HTTP handlers: stream responses where reasonable

[julianbrost]

After we are done laying the foundations for streaming JSON in HTTP responses (see #10409 and #10516), we should go over all the HTTP handlers an check where it makes sense to use the functionality.

We should not only consider memory usage but also overall runtime of the individual handlers in the sense that so far, the response is only sent after the handler has finished, i.e. the time to the first byte of the response can be quite high. Having a continuous response stream instead of a burst after handling the response is nicer for the client given that there's a regular sign of aliveness. (Or in other words: it's not very nice if you send a request, then there's 30 seconds of complete silence, and then only then you receive a response.)