Evaluate and switch of EC chain

[jonastheis]

Nodes periodically commit to the accepted contents of epochs. Whenever a different commitment is observed on the Tangle, the node has to evaluate the weight of the competing chain by obtaining a list of validators and a proof of their weight. Whenever such competing chain is found to have accrued more weight than the one known to the node, the ledgerstate should be rolled-back to the forking point and the Tangle resynced using the new chain.