Use indexed rather than named Set() when converting to v8::Array

The meat of this change is in the Array case of mongoToV8Element.
mongoToV8() was removed since this was the only caller.
Speed up conversion of v8::Arrays to BSON

Two major improvements:
* Use BSONObjBuilder::numStr() to convert index to string (has global
     cache of strings for 0-99)
* Use indexed rather than named iteration over the v8::Array
SERVER-9267 Make readonly v8 objects use same getters as regular lazy objects

Also fixes SERVER-9251