Single Query Loading loads as the name suggests complete aggregates using a single select.
While the ultimate goal is to support this for all aggregates, this commit enables it only for simple aggregate and also only for some operations.
A simple aggregate is an aggregate that only reference up to one other entity and does not have embedded entities.
The supported operations are those available via `CrudRepository`: `findAll`, `findById`, and `findAllByIds`.
Single Query Loading does NOT work with the supported in memory databases H2 and HSQLDB, since these do not properly support windowing functions, which are essential for Single Query Loading.
To turn on Single Query Loading call `RelationalMappingContext.setSingleQueryLoadingEnabled(true)`.
Closes#1446
See #1450
See #1445
Original pull request: #1572
93821f5f42