Better detection of Spring Session store

With the modules split in Spring Session 2, there is a high chance that the project’s classpath contains only one store implementation.

We should improve our auto-configuration structure to switch to that store in that case (and that case only) so that setting the property is not required.

There are two failures scenario to handle:

• More than one implementation is available: report that the store-type parameter is not set and which implementations are current available
• No implementation is available (that could happen for those migrating and having only spring-session-core on their classpath that used to contain the core implementations). Perhaps reusing the NoSuchBeanDefinition failure analyzer could be a good idea as it would tell why the stores were not configured. If Spring Session provides a way to discover the available implementations (as Spring Data does with spring.factories ) we could build a more dedicated error message.

/cc @rwinch @vpavic

[snicoll]

Failure analyzer for the first case

***************************
APPLICATION FAILED TO START
***************************

Description:

Multiple Spring Session store implementations are available on the classpath:
    - org.springframework.session.jdbc.JdbcOperationsSessionRepository
    - org.springframework.session.hazelcast.HazelcastSessionRepository


Action:

Consider any of the following:
    - Define the `spring.session.store-type` property to the store you want to use
    - Review your classpath and remove the unwanted store implementation(s)

[snicoll]

Closed by https://github.com/spring-projects/spring-boot/commit/4670cc779548677c5286fa5f1a5beee9b08a61eb