Rebuild index async on boot

[joepio]

When Atomic-Server starts with --initialize, we rebuild the index. Building the index is currently a blocking operation. This means Atomic-Server is offline while it is indexing. This means potential downtime for users. We could probably run the index command in a separate thread.

Doing this async means that it can happen that some Queries will return the 'wrong' results. This might break a test or two.