Fix for #3012 Manual server shutdown leads to memory leak

The issue being that shutdown() // with no args was
not calling ShutdownManager.unregisterDatabase(this)
noting that shutdown(boolean, boolean) did.

This change merges the old shutdownInternal(boolean, boolean)
method into shutdown(boolean, boolean) and simplifies
shutdown() to just call shutdown(boolean, boolean).

ebean-api/src/main/java/io/ebean/event/ShutdownManager.java

@@ -143,7 +143,7 @@ public final class ShutdownManager {
       }
       // shutdown any registered servers that have not
       // already been shutdown manually
-      for (Database server : databases) {
+      for (Database server : new ArrayList<>(databases)) {
         try {
           server.shutdown();
         } catch (Exception ex) {