Ensure the search writer is free from any threading issues

[alexmic]

According to the documentation:

Because opening a writer locks the index for writing, in a multi-threaded or multi-process environment your code needs to be aware that opening a writer may raise an exception (whoosh.store.LockError) if a writer is already open. Whoosh includes a couple of example implementations (whoosh.writing.AsyncWriter and whoosh.writing.BufferedWriter) of ways to work around the write lock.