remove potential infinite loops

[alokmenghrajani]

When the backend is processing data and something goes wrong, the request is re-queued (with the hope that another node will successfully handle the work). This creates a potential infinite loop. You can see this happen if you try to process an address with a huge number of transactions using the Electrum backend (e.g. 18cBEMRxXHqzWWCxZNtU91F5sbUNKhL5PX has 12k transactions, too many for Electrum).

When we re-queue the work, we should track how many attempts we have done. If the number of attempts is too high, we should panic().