Deleting pending handling can cause OOM

> Mike:
>> Could happen when doDeleting the pending docs too. James: try
>> sending commit every 500k docs or so.

Yonik:
> Hmmm, right... some of the memory usage will be related to the treemap
> keeping track of deleted items, and some of it will be related to the
> reader itself (the term index, and the norms).

> Perhaps we should have some sort of limit on the number of deletions
> we cache so people don't have to worry about that part.

Good idea. Perhaps a solrconfig.xml setting, <maxPendingDocs>? Default to 100k?