[mongodb] return OK on updates with w=0

[kruthar]

in #746 it was found that when using the mongodb-async binding with a write concern of w=0 then update operations will always return -1 which in turn reports an operation status of NOT_FOUND.

Since you cannot tell what the result of the update operation is whne you fire and forget, it is reasonable to just return success for all update operations in the mongodb-async binding when using a write concern of w=0.

[busbey]

would BATCHED_OK be more appropriate? @allanbank, have an opinion?

I'm +1 with the change as is, or with BATCHED_OK instead.

[allanbank]

@busbey - I think the OK status is a closer fit than BATCHED_OK. The operation is successful to the level of durability that the user requested.

[busbey]

Works for me!