Use an existing Lucene index directly as a CacheLoader source

It's pretty easy to copy a Lucene index into Infinispan, but it would be much nicer to just have it point to an existing index on a filesystem and use Infinispan as a thin layer on top of it; this would also allow for easy replication of changes.

This is initially going to be a CacheLoader only, so meant for a read-only index. Shouldn't be hard to adapt the logic to allow for write-through in the future.