Remove DictionaryTerm with count 0 during compact (workaround for #374)

[mmindenhall]

I spent a couple hours on this last night as a workaround for #374.

The implementation removes all DictionaryTerm entries with Count=0 from the index, in configurable batches, within a transaction. Originally I did this all in one large transaction, but settled on this approach to avoid locking out other writers for an extended period of time. A batch size of 250 seems like a good default number for a server-based implementation...I'll be using batch sizes of more like 50-100 in my app (running on a single core ARMv7).

I ran this test, that I wrote before submitting #374:

1. Create and initialize a new index, take snapshot of size of index folder
2. Do the following 5 times:
3. Add 1000 documents to the index
4. Take snapshot of size of index folder
5. Delete all documents from the index
6. Call the new Compact method I added in Add compact method to goleveldb store #373
7. Take snapshot of size of index folder

I verified that there were no dictionary term entries present after the test, and the .ldb file contained only the document mapping and fields.

I also ran a variation of the test above to ensure that documents could still be indexed during the execution of CompactWithBatchSize. Just before calling CompactWithBatchSize, I started another goroutine to index documents during the compact. I verified that the documents were indexed, document count after finishing was correct, and there were no dictionary term entries with count 0.

[mmindenhall]

Yes...definitely makes more sense that way. I made the change, and ran my tests again.