ConsentStore's StringifyScopes method should use StringBuilder

[pixelbadger]

StringifyScopes currently uses a LINQ Aggregate to generate a concatenated, comma-separated list of strings for persistence in the EF model. When compared to a StringBuilder implementation this approach is both slow and risks high memory allocation under heavy load.

I've implemented an alternative in this fork, which includes some performance tests demonstrating the improvement. If this is considered a worthwhile improvement, I can send a pull request.

[brockallen]

Please send a PR to dev.