Add consensus block size limit (#111, from #110)

CheckBlock() validated block size against MAX_SIZE, the serializer's generic 32 MB guard rather than a block rule; MAX_BLOCK_SIZE did not exist in this tree at all, because Bitcoin's 1 MB cap arrived in 0.3.x, after the 0.1.0 this fork came from. A miner could produce a 32 MB block and every node would download, verify and store it.

Original work by mtg0001 in #110, rebased over #109 with the self-test conflict resolved, plus a comment explaining why the inherited count-against-bytes test stays.

Measured before choosing the number: 5709 blocks read from blk0001.dat give a largest block of 34,290 bytes and a median of 216, so 1 MB leaves 29x of headroom and rejects nothing that exists. A node built from this branch then synced genesis to 5706 on the live network with zero size-limit rejections and zero CheckBlock errors -- the same test that caught BIP30 rejecting a real block at height 859.

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