Improve cleanup of inactive transactions from threadLocal when no end() used

[rbygrave]

So when we miss off a transaction.end() the thread local state holds an inactive transaction. This isn't a problem people will generally notice as inactive transactions are handled when the next transaction occurs.

However, we can do better and cleanup the threadLocal state on both commit() / rollback() as well as end().

Steps to reproduce

    Transaction txn1 = server().beginTransaction();
    try {
      txn1.commit();
    } finally {
      // accidentally omit end()
      //txn1.end();
    }

    // internal check on thread local status ... application code doesn't do this
    SpiTransaction txn2 = DefaultTransactionThreadLocal.get("h2");
    assertThat(txn2).isNull();

So with this change Ebean will perform a cleanup of transaction scope on commit() / rollback() ... and also on end() - and will perform the cleanup once so usually on commit() and rollback().

Internals note that ScopedTransaction handles nesting transactions so we really only do the scope cleanup when we are back at the top (no nested transactions to pop).