Thread per-statement error information from WriteResult::results to the matching key. This should be in performInserts, performSingleUpdateOp, performDelete, and populateReply

Refactor mongod’s write registration to use the per-opIndex map. This is because mongod currently holds one query stats key at a time, which works today because it records stats immediately after each successful statement. We need more than one slot (need a map) in order to look up “the key for statement N” when a writeErrors entry arrives. 

Write tests for insert/update/delete for ordered and unordered writes