v0.25.1: Fixed DNS responses, correctly handle many missing txs, fix a DoS on empty block

Major improvements since v0.25.0:

DNS: Name registry now returns the question with the answer

The name registry was mistakenly not including the question whenever it was
answering a DNS query, in violation of the specification. This is now fixed.'

DNS: Responses will now include the proper RCode

Previous response could include a valid answer along with a FormErr RCode,
instead of the correct NoError. The RCode is now set correctly.

Rewards: Do not crash on very short block interval

Using block intervals of exactly 5 seconds would crash the nodes at the payment interval,
due to a division by 0. This has been corrected.

Prevent nodes getting stuck when there are too many unknown transactions

When the nodes was trying to fetch unknown transactions, if there were too many of them,
the call will fail, and the node would fail to retry with a lower limit.
The call is now always bounded to a reasonable value.

Do not crash when looking up a merkle path in an empty block

When calling the API to look up a merkle path inside of an empty block,
the node would assert and shut down before verifying the hash.
This out-of-order bug has been fixed and the node now correctly handles this situation.