SERVER-17158 add delimiter to planCacheKey for sort args

[jameswahlin]

The planCacheKey has no delimiter on sort arguments. This can lead to 2
different sets of sort arguments having the same key. This patch adds the
delimiter to address.
Before:
db.coll.find({a: 1}).sort({a: 1, b: 1}) => eqaaaab
db.coll.find({a: 1}).sort({aab: 1}) => eqaaaab
After:
db.coll.find({a: 1}).sort({a: 1, b: 1}) => eqaaa,ab
db.coll.find({a: 1}).sort({aab: 1}) => eqaaaab