Minimize MapSession's use of Secure Random

Previously when creating a MapSession from an existing session required
that UUID.randomUUID() be invoked. This could slow down the system
since it requires entropy.

MapSession now has a constructor that accepts the id which prevents
Secure random from being used when the session is already known.

Fixes gh-271

spring-session/src/test/java/org/springframework/session/MapSessionTests.java

@@ -34,7 +34,7 @@ public class MapSessionTests {
 
 	@Test(expected = IllegalArgumentException.class)
 	public void constructorNullSession() {
-		new MapSession(null);
+		new MapSession((ExpiringSession) null);
 	}
 
 	/**