DATAKV-87 - Allow definition of target Map type via @EnableMapRepositories.

We added the configuration attribute ‘mapType’ to @EnableMapRepositories which allows to change defaulting of the Map structure used by the MapKeyValueAdapter. To do so we now register a MapKeyValueAdapterFactory and delegate creation of the adapter to it. This also allows usage of interface types such as plain java.util.Map which will initialize the adapter with the approximated type resolved via CollectionFactory.

Original pull request #2.
DATAKV-87 - Polished implementation of configurability of Map type on @EnableMapRepositories.

Removed the MapKeyValueAdapterFactory in favor of using instantiating MapKeyValueAdapters directly. Reverted additional hook in KeyValueRepositoryConfigurationExtension as we now rather use inner bean definitions.

Original pull request: #2.
DATAKV-93 - Added README and reference documentation.

Original pull request: #3.