Perform JSON-encoding of CS API responses in a background thread

[richvdh]

This is an implementation of the solution proposed in #6998 (comment); viz. moving JSON-encoding out to a background thread.

[richvdh]

I'm not entirely sure I'm happy with this.

I worry that it might be making things substantially worse for the common case: if you've got something trivial to encode, then pushing it out to a separate thread is likely to be more work than just doing the encoding inline.

I also worry that sharing the default threadpool with whatever else might be going on there (media thumbnailing, password hashing) could mean that we end up blocking C-S responses behind other CPU-intensive work.

Still, I wrote it to get my HS going after riot-android decided to DoS it with initialsyncs, so I thought it was worth opening a PR.

[richvdh]

Per #6998 (comment): I think there are better ways to solve this.