Reduce mutex contention around host_source and ring refresh concurrency

[jon-whit]

There's an open PR to fix some mutex contention around the host_source and ring refresh concurrency that @nanassito contributed. See #1944.

Here's a mutex blocking profile taken from a production runtime sample under load.

I know that this doesn't account for all of the mutex contention, but it's still a decent percentage. Can we get some attention on that PR and have a maintainer take a look?

[jon-whit]

@joao-r-reis or @worryg0d it appears ya'll are the two most active maintainers. Tagging you for visibility.