Make Db_Mysql::insertManyAndExecute support sharding

[EGreg]

Right now, Db_Query_Mysql objects support sharding, but we also need to be able to do this with insertManyAndExecute, by setting the criteria for each row that's going to be inserted, and then running them through the shard function, aggregating into queries and returning those queries on the appropriate shards.

Good luck :)