Multiple replicas assigned to a task

[quellobiondo]

In MRSG the only replicas that can be assigned to a task are the ones to Stragglers.

• [x] Track which task a node has already completed (not needed in the original MR)
• [x] Replicate the same task to X nodes
• [x] Threshold for completion of K nodes (K<X)
• [x] Support for multiple intermediate map-task result
• [x] Default scheduling adaptation
• [x] Speculative copies adapted to the replicas