BUG: fix nonce for concurrency tasks

[lacabra]

Continues the work initiated in #181, this PR implements the true intent of that first PR, tracking locally nonces used that are submitted concurrently too quickly before the contract can update them. The exception is the Contract Deployment where the nonce has to be the "true" nonce in sync with the contract, otherwise the contract will throw an error. In all other cases (compute task), the nonces do not need to be in sync.

Please note that the contract and EnigmaJS DO NOT support concurrency in contract deployments, only in computations.