This commit restructures configuration support for Redis-backed
HttpSession with aim to enable users to easily select the
SessionRepository implementation they prefer to use.
This is achieved by introducing [at]EnableRedisIndexedHttpSession
annotation that can be used to configure RedisIndexedSessionRepository,
while the existing [at]EnableRedisHttpSession will going forward
configure RedisSessionRepository as the SessionRepository implementation
used by Spring Session.
Additionally, this also introduces AbstractRedisHttpSessionConfiguration
as the base configuration class that manages common aspects of
Redis-backed HttpSession support, which is then extended by more
specific configuration classes that provide specific SessionRepository
implementation.
Closes gh-2122
