Make dataMapper not a singleton
In order to make further refactoring for https://review.typo3.org/#/c/53974/ where we want to pass a query/query settings object to datamapper, first we need to make dataMapper not a singleton.
All dependencies injected in datamapper are singletons.
There is a property "dataMap" which serves as a runtime cache for datamap.
btw, Datamap is also cached additionaly in dataMapFactory.
so the datamap property can be either replaced by a runtime cache from core, or kept as is, or moved to DataMapFactory