[#937] Fixed nested transactions that query database.

Nested transactions, Transaction#executeOther(Transaction), where the
inner transaction performed a query ran into NPE since the Database
used for the query had not been defined.

Plan/common/src/main/java/com/djrapitops/plan/db/access/transactions/Transaction.java

@@ -140,9 +140,11 @@ public abstract class Transaction {
     }
 
     protected void executeOther(Transaction transaction) {
+        transaction.db = db;
         transaction.connection = this.connection;
         transaction.performOperations();
         transaction.connection = null;
+        transaction.db = null;
     }
 
     protected UUID getServerUUID() {