This allows nodes to handle confirm_req and confirm_ack messages with payload of more than 16 hashes, up to 255. During testing it was shown that fewer, larger messages provide considerable improvement in performance, especially on CPU limited hardware. This PR is one of the steps to better align our message formats with this philosophy.

What's important to note is that, even with ability to handle large votes, nodes will continue to generate vote requests & responses with legacy rules (<= 12 hashes per message) to keep compatibility with previous versions. Once majority of the network upgrades, switching to larger votes should be as simple as modifying two constants in code.

Header before:
+-------+-----------+--------+
|  1-4  |    5-8    |  9-16  |
+-------+-----------+--------+
| count | block type | unused |
+-------+-----------+--------+

Header after:
+---------------+-----------+---------------+--------+---------+
|      1-4      |    5-8    |     9-12      | 13-15  |   16    |
+---------------+-----------+---------------+--------+---------+
| count (upper) | block type | count (lower) | unused | v2 flag |
+---------------+-----------+---------------+--------+---------+