CachedLDAPConnection: Don't cache fields we don't use

CachedLDAPConnection: Don't cache fields we don't use

I had previously said:
> It would be nice to automatically detect and drop any non text
> fields, but that's probably not worth the trouble either...

Since the configuration ID is in the cached key, we already know
exactly which fields we need to save in the cache.  Only caching those
fields avoids binary fields and fields with large values (jpegPhoto is
both).  I made the cached fields configurable because the
CachedLDAPConnection instance doesn't know about the fields needed by
format_entry() and format_columns().  If you don't configure the
cached fields, a minimal default is setup in the main block.