Fixes #4290: adds volatile metadata cache, unit tests, and pre-loading for fetched entities
fixed: elgg_get_site_entity now works as documented
Merge pull request #387 from jeabakker/#4868

#4868 - possible unexpected result from elgg_get_site_entity
Merge pull request #377 from mrclay/4290-1.8

Fixes #4290: adds preloading volatile metadata cache and unit tests
Merge commit 'ca0f22f098bdac6982f30156884cf9f4c498c689' into tmp_merge
Merge commit '7239da3fb83bbfdd414f96339ef33fee4d68554e' into tmp_merge