Fixes https://github.com/element-hq/synapse/issues/18659
This changes the Tokio runtime to be attached to the Twisted reactor.
This way, the Tokio runtime starts when the Twisted reactor starts, and
*not* when the module gets loaded.
This is important as starting the runtime on module load meant that it
broke when Synapse was started with `daemonize`/`synctl`, as forks only
retain the calling threads, breaking the Tokio runtime.
This also changes so that the HttpClient gets the Twisted reactor
explicitly as parameter instead of loading it from
`twisted.internet.reactor`
f031105eee