Reconsider the choice of Redis driver in `spring-session-data-redis`

[vpavic]

Our Data Redis module (spring-session-data-redis) currently pulls in Jedis.

With Lettuce getting more attention recently which includes the first-class support in Boot (see spring-projects/spring-boot#5311) and potentially becoming the default Redis driver in Boot's spring-boot-starter-data-redis (see spring-projects/spring-boot#9536) it's an obvious choice for our Data Redis module. We already use Lettuce instead of Jedis in some of our sample apps.

The third option is not to include Redis driver at all in our Data Redis module and leave the choice to user. The rationale behind this is that we don't actually configure RedisConnectionFactory but rather expect it provided to us (see RedisHttpSessionConfiguration:109) - following that it might makes sense for us not to pull in a Redis driver and leave that choice to user to either declare the desired Redis driver as first-level dependency or rely on Spring Boot to provide dependency for the driver (since it actually configures it).