Allow multiple nodes to compute same subtask.

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Implement redundant verification from: https://github.com/golemfactory/golem-rd/wiki/Verification

First approach (abandoned branch) was here: https://github.com/golemfactory/golem/tree/redundant_computation

One potential approach is described below, but any other rational algorithm will be fine.

Algorithm proposal:
We define replication level x and consensus level c > x/2 and max tries m. Tasks send up to x copies of subtasks to different nodes and wait for the results. If c same answers is received the subtask is verified and all answers that are consistent with the consensus are accepted. If consensus is not reached within x subtasks, than additional m - x tasks may be send. If consensus is not reached subtask is marked as failed.
If consensus is reached before all x subtasks are send, then subtask is accepted.

Necessary changes:

• subtask verification may be delayed
• task should have num_subtask parameter
• total_tasks = m * num_subtasks
• task may be computed before total task number is reached

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It has been implemented for Usage Market and WASM.