add progress bar during shuffling / decryption

[ineiti]

When having 1000 votes or more, the shuffling / decryption / combination phases take between 30 seconds and 3-5 minutes each. For this timeframe it would be nice to have a progress bar. Some ideas what should be in there:

• which step the system is in (which nodes already shuffled)
• warnings and errors produced in the logs of the nodes
• perhaps show an ETA, either based on statistics of n votes, or with some magic from the ledger