Unstable queue and/or worker in Redemption Server

[serial-coder]

Some kind of random errors needed to be investigated by logs or code review.

• Sometimes, a user had to wait for long and got "Found error" message.

• Sometimes, Redemption server's queue seemed to be not working properly and need to be cleared. For example, sometimes if a user types in a valid VoucherID but invalid ParityCode for the first time, the server will send tx to the blockchain (of course, the tx will fail). Later on, if a user re-redeems again with both valid VoucherID and ParityCode, the new tx gets stuck in the queue or at the worker somewhere and the server will not send a new tx for that VoucherID anymore.