Update mmap preload usage and config

[aprudhomme]

The new version of lucene lets you specify a predicate for determining which files should be preloaded by an MMapDirectory. This lets us simplify our usage and remove the SplitMMapDirectory.

Preloading is now compatible with using FSDirectory. If the directory implementation is an MMapDirectory, it will get configured for preloading.

Additionally:

• The config has been updated to use the 'preload.` prefix. This will make it cleaner to add more properties (for example, using the context as part of the decision)
• The default value is now false. This feature has some large resource implications and should be deliberately enabled. Since we default to FSDirectory, preload would have been disabled by default prior to this change.