Configuration validation for Metadata Cache in Lucene Directory

from ispn-dev:
Proposal:
let's be more aggressive on validation.

• make preload mandatory (for the metadata only)
• eviction not allowed (for the metadata only)

Note that I don't think we're putting much of a restriction to users, we're more likely helping with good guidance on how these caches are supposed to work.
I think it's a good thing to narrow down the possible configuration
options and make it clear what we expect people to use for this purpose.