Offline training mode for schema guessing

Our data driven schema guessing doesn't work under many situations. For example, if the first document has a field with value "0", it is guessed as Long and subsequent fields with "0.0" are rejected. Similarly, if the same field had alphanumeric contents for a latter document, those documents are rejected. Also, single vs. multi valued field guessing is not ideal.

Proposing an offline training mode where Solr accepts bunch of documents and returns a guessed schema (without indexing). This schema can then be used for actual indexing. I think the original idea is from Hoss.

I think initial implementation can be based on an UpdateRequestProcessor. We can hash out the API soon, as we go along.