Add has_merging_threads function to IndexWriter

[kryesh]

Adding a function to IndexWriter that returns true if there are merges running and false otherwise, this is useful for applications that have multiple indices that may want to shut down idle writers to conserve memory.

This is a cleaner solution to that idle index use case than running wait_merging_threads since calling that blocks until the merges are done while keeping the index locked, This increases latency (sometimes considerably so depending on the merge size) if you need to create a new writer because the index is no longer idle.