Revert "User Oj gem for serializing Hash to JSON."

This reverts commit 579e7632af51085fadbe0b6318107ae660b3f28a.
serialize id as a string, not BSON
stringify keys

For Oj dump to be backward compatible
Revert "Revert "User Oj gem for serializing Hash to JSON.""

This reverts commit c503983ba21dd36901fff3f1b2cefd090a71198c.
Merge branch 'faster-serializer'