Add server.session.store-dir support

Add support for a `server.session.store-dir` property which can be used
to specify where session data source be saved.

Fixes gh-4191

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -87,6 +87,7 @@ content into your application; rather pick only the properties that you need.
 	server.display-name= # the display name of the application
 	server.use-forward-headers= # if X-Forwarded-* headers should be used (default is off unless running in a known cloud)
 	server.session.persistent=false # true if session should be saved across restarts
+	server.session.store-dir= # dir used to save session data
 	server.session.timeout= # session timeout in seconds
 	server.session.tracking-modes= # tracking modes (one or more of "cookie" ,"url", "ssl")
 	server.session.cookie.name= # session cookie name

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -3584,7 +3584,7 @@ Spring Session provides support for managing a user's session information. If yo
 writing a web application and Spring Session and Spring Data Redis are both on the
 classpath, Spring Boot will auto-configure Spring Session through its
 `@EnableRedisHttpSession`. Session data will be stored in Redis and the session timeout
-can be configured using the `server.session-timeout` property.
+can be configured using the `server.session.timeout` property.