Not thread safe, caching issues

[sqrrrl]

The validator itself isn't thread safe due to the use of @problem to report back status. This requires creating a new instance each time (or at least being very careful in other ways.) Creating multiple instances however partially defeats the built-in caching of certificates.

Could use a refactoring to better separate out the certificate loading/caching from the validator. It's partially there in that a list of certs can be passed in to the validator, but that leaves a lot of implementation in the hands of users of the lib.

Should be a more complete solution than what is offered.

[gguuss]

Resolved in this PR we should add tests and close when verified.

[hxiong388]

While #10 definitely improves the thread safety by removing the @problem instance variable. But it's still not completely thread safe because it still uses Hash and Hash#merge! to cache the certificates.