[keycloak-dev] Hybrid store committed

I've committed the hybrid store including jpa store implementations. It's enabled by default for the testsuite and the server, using jpa for realms and users, and mem for sessions. By default the JPA stores all use the same persistence-unit, but it's possible to split into multiple persistence-units and configure through keycloak-server.json if someone wants to for example store realms in one db and users in another. Remaining work includes: * Mongo stores * Updates to Keycloak session - remove model methods and add an enlist transaction feature for providers * EntityManager and Mongo connection providers - I'm going to add providers for retrieving EntityManager instances and Mongo clients. The main reason for this is so that multiple stores can share the same connection/transaction when possible * Delete JPA and Mongo model providers * Update tests