Threadsafe cache

[ssande-fwd]

Fixes #165

You can test using this program https://github.com/ssande-fwd/mkc-concurrency-test
You can see the error output I would get every 20 or so compilations in the readme for that program.

[ssande-fwd]

@abchatra this is starting to become important for MicroChat. Can you have somebody review this? I would like to avoid creating a fork.

[ssande-fwd]

I just ran an AI audit since it's been a while since I wrote this. Apparently this operation is redundant because renameAsync will overwrite an existing file, but I haven't tested.
if (await host().existsAsync(filePath)) { await host().unlinkAsync(filePath); }