TooManyWitnesses RAD error tally

[drcpu-github]

This is an alternative implemenation to #2444 where instead of refusing to build a block with a data request requiring too many witnesses, the data request is included by a miner together with a tally. The tally has a TooManyWitnesses Radon error as result.

This is potentially more robust because of two reasons:

1. It won't keep invalid data requests in the transaction pool
2. By creating a tally the (decentralized) bridge can report an actual tally result rather than make something up