Share mutex between all Ohm::Model classes to avoid bad mix of data on multi-threading execution

With current approach of separated of mutex per Ohm::Model class, while they still use the same redis connection, when data is being loaded by different model in a multi-threading environment, bad mix of data from 2 or more models is returned for a model to handle, which leads to problem. This pull request forces all Ohm::Model sub classes to share the same mutex to solve that issues. The fix is just 1 character('@') and the rest is just a way to craft exception to communicate more details to developer to get an understand of what's going on.