[keycloak-dev] JPA connections provider and multiple transaction support

To prevent creating multiple EntityManager's for a single request I've added a JPAConnectionProvider that is used to share an EntityManager instance between multiple providers. https://github.com/stianst/keycloak/blob/master/connections/jpa/src/main/... https://github.com/stianst/keycloak/blob/master/connections/jpa/src/main/... In the future we could consider adding support for multiple persistence-units, but for now I think we should have a limitation that all JPA providers use the same database/persistence-unit. I've also added some very basic support to KeycloakSession so providers can enlist their transactions with the session: https://github.com/stianst/keycloak/blob/master/model/api/src/main/java/o... https://github.com/stianst/keycloak/blob/master/model/api/src/main/java/o... The idea being that a provider when used can enlist its transaction with the session. I've not committed any of this to master as I wanted to have everyone's opinion on it first.