[OP] Parallel dispute handling

[ggwpez]

The operator is currently unable to handle more than one challenge at once.
The two parallel handleChallenge calls race and one of the transactions can end up with a wrong account nonce resulting in this error:
ERRO[0070] Operator.handleChallenges: Failed to handle challenged event ChallengedEvent{Account: 0x22d491Bde2303f2f43325b2108D26f1eAbA1e32b, Epoch: 4}: sending challenge response: the tx doesn't have the correct nonce. account has nonce of: 3 tx has nonce of: 2

To enable parallel dispute testing, replace testChallenge(t, clients[0]) with testChallenge(t, clients...) in integration_test.go:76 and run the integration test.