Relay: manage client/dapp reputation

[drortirosh]

The relay should maintain a list of rejected calls (that is, relayed calls in which accept_relayed_call returned "true" when called off-chain, but returned an error on-chain, and thus caused the relay to pay for the transactions)
The relay should not completely reject such dApps, since there are edge-cases that might cause such cases.
It should give it some weight, so that a dApp that does it repeatedly will stop getting service.

Implementation depends on #94

[drortirosh]

• Periodically, scan for past TransactionFailed (failed transactions, that caused the relay to pay)
• For each such target contract, check how many valid TransactionRelayed events
• The logic is: The ratio of good/failed should be above a given factor.