Clean up thread_locals in EOS VM OC

[linh2931]

Please see the comments by @spoonincode in (#1498 (comment))

(obviously not in this PR) but I'd really like for us to noodle on how to move away from these thread_locals, they have dangerous lifetime semantics imo. In this particular example, when eosvmoc_tier is destroyed, cc is destroyed, but the exec created with a reference to that cc lives on. So if executor's dtor (which will run well after eosvmoc_tier's dtor) touches the code_cache_async that was passed to it via reference: bad news! It's a real trap -- passing a reference to some object to the ctor of an object implies that reference will be valid for the lifetime of the object.. but it's not true here.