Restructure Redis HttpSession configuration support

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

spring-session-docs/modules/ROOT/examples/resources/docs/HttpSessionConfigurationNoOpConfigureRedisActionXmlTests-context.xml

@@ -10,7 +10,7 @@
 
 	<context:annotation-config/>
 
-	<bean class="org.springframework.session.data.redis.config.annotation.web.http.RedisHttpSessionConfiguration"/>
+	<bean class="org.springframework.session.data.redis.config.annotation.web.http.RedisIndexedHttpSessionConfiguration"/>
 
 	<!-- tag::configure-redis-action[] -->
 	<util:constant

spring-session-docs/modules/ROOT/examples/resources/docs/http/HttpSessionListenerXmlTests-context.xml

@@ -14,7 +14,7 @@
 
 
 	<context:annotation-config/>
-	<bean class="org.springframework.session.data.redis.config.annotation.web.http.RedisHttpSessionConfiguration"/>
+	<bean class="org.springframework.session.data.redis.config.annotation.web.http.RedisIndexedHttpSessionConfiguration"/>
 
 	<bean class="docs.http.AbstractHttpSessionListenerTests"
 		factory-method="createMockRedisConnection"/>