Dependency registration is not thread-safe

[KevinBrowne]

Sinject::Conatainer#register does a check and set that is not protected in a critical section, making it vulnerable to race conditions.

Broadly, the method looks like this:

def register
  raise 'Duplicate key' if key_exists?

  do_registration
end

If you attempt to register more than one dependency with the same key, the #register method should raise a duplicate key error. However, two concurrent calls to #register can both pass the duplicate key check, and proceed to the registration step, with the last one "winning". It would be better for the container to always raise the error rather than behave unpredictably, however rare the circumstances.

This is difficult to reliably reproduce.