Fix session event handling in HazelcastSessionRepository

Previously, invoking HttpServletRequest#changeSessionId on session backed by HazelcastSessionRepository generated generated invalid session destroyed and session created events. This was due to use of IMap#remove and IMap#set when handling the change session id.

This commit improves change session id handling to prevent publishing invalid events by using IMap#delete instead of IMap#remove and keeping track of originally assigned session id.

Closes gh-1077

spring-session-core/src/main/java/org/springframework/session/MapSession.java

@@ -122,7 +122,12 @@ public final class MapSession implements Session, Serializable {
 		return this.id;
 	}
 
-	String getOriginalId() {
+	/**
+	 * Get the original session id.
+	 * @return the original session id
+	 * @see #changeSessionId()
+	 */
+	public String getOriginalId() {
 		return this.originalId;
 	}