Conditionally reset the transaction isolation level based on whether the user has altered it or not. Turns out it is expensive for some databases.

src/main/java/com/zaxxer/hikari/proxy/ConnectionProxy.java

@@ -71,6 +71,7 @@ public abstract class ConnectionProxy implements IHikariConnectionProxy
 
         creationTime = lastAccess = System.currentTimeMillis();
         openStatements = new FastStatementList();
+        isTransactionIsolationDirty = true;
     }
     
     public final void unregisterStatement(Object statement)