Switch to lettuce Redis driver

[mp911de]

Jedis requires a connection per concurrent request. The number of connections grows with the number of concurrent requests which can exhaust connection limits.

lettuce is a thread-safe driver that allows operating multiple threads with one connection. Spring Cloud Connector will provide with 1.2.4 configuration support for lettuce.

Depending on the experience with lettuce on PCF, we should also update the Spring Session docs and add examples how to configure Spring Session with lettuce.