Send request level commitWithin as a param rather than setting it per doc

In SolrCloud the commitWithin parameter is sent per-document even if it is set on the entire request.

We should send request level commitWithin as a param rather than setting it per doc - that would mean less repeated data in the request. We still need to properly support per doc like this as well though, because that is the level cmd objects support and we are distributing cmd objects.