Fix database configuration when setup through session

[fixes #3335]
eager load part 1

Adding mock preloaders to eager load entries.
eager load: first batch on tests
eager load: part 2

- adding tests
- ensure that belongs_to, has_one, has_many work
- many to many still broken
eager load: part 3

 - use set_relation on eager_load
 - make many_to_many work
:heart: :two_hearts: :green_heart: tests
first eager_load cleanup
eager load: adding more tests
eager load: more tests
Adding query count tests for eager_load
rm eager_load
Add more tests for eager_load on HasMany relations
Raise when try to eager_load a polymorphic belongs_to
Add more tests for eager_load on belongs_to relations
Small re-order on has_many_spec
Add more tests for eager_load on many_to_many relations
remove polymorphic pending test
Add more tests for eager_load on has_one relations
Rename and bit of cleanup on eager_load
Merge pull request #3279 from mongoid/eager_load

Eager load ! ! !