TransportLayerASIOTest is the last user of ThreadContext.

It's the only reason we have it. And because we have it, we have to have a ThreadContext::get call in every thread we spawn (and we'll still miss some threads).

It's a flaky test and what it's using ThreadContext for can be done with dependency injection, more reliably. Then we can have a better test and proceed to get rid of ThreadContext (per SERVER-66456).