Allow persistent servlet sessions across restarts

Update Tomcat, Jetty and Undertow to serialize session data when the
application is stopped and load it again when the application restarts.

Persistent session are opt-in; either by setting `persistentSession`
on the ConfigurableEmbeddedServletContainer or by using the property
`server.session.persistent=true`.

Fixes gh-2490

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

@@ -78,6 +78,7 @@ content into your application; rather pick only the properties that you need.
 	server.jsp-servlet.registered=true # Whether or not the JSP servlet is registered
 	server.servlet-path= # the servlet path, defaults to '/'
 	server.display-name= # the display name of the application
+	server.session.persistent= # true if session should be saved across restarts
 	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

@@ -1581,7 +1581,7 @@ Common server settings include:
 
 * `server.port` -- The listen port for incoming HTTP requests.
 * `server.address` -- The interface address to bind to.
-* `server.sessionTimeout` -- A session timeout.
+* `server.session.timeout` -- A session timeout.
 
 See the {sc-spring-boot-autoconfigure}/web/ServerProperties.{sc-ext}[`ServerProperties`]
 class for a complete list.