Search across multiple caches

This would conceptually mean moving the SearchManager from Cache scope to CacheManager scope, which would allow us to share HS search factory. Would also need to add indexing configuration at cache manager level.

Having a SearchManager at cache scope would still be desired for cases where the user needs to configure particular indexing settings for certain caches.

Regarding indexes, we could have one index per class or one index per class + cache name; remains to be decided.

See discussion here: http://markmail.org/message/oe4jeq4k73f44rts