[DDC-1398] Extra-lazy get for indexed associations

If an association is EXTRA_LAZY and has an indexBy, then
you can call get() without loading the entire collection.
Return NULL for non-existent keys

The load() function already returns just one entity or NULL, so
the current() is not needed and the result can be returned directly.
Test actual data returned by get()
Get rid of variable
Use find() if the indexBy field is the identifier
No need to lookup metadata
Extra test for indexBy identifier versus indexBy other fields
Merge pull request #706 from sandermarechal/extra-lazy-get

[DDC-1398] Extra-lazy get for indexed associations