Currently it is quite easy to introduce concurrency issues while working
with transaction object. For example, race issue will exist every time
while transaction is processed in a separate goroutine and caller will
try to check for an error before event to Done channel is sent.
This change removes all the data that is updated on transaction and leaves
it with ID, Args and Context (which is not used at the moment).
Signed-off-by: Dmitry Shulyak <yashulyak@gmail.com>