consensus: cap block size at 1 MB, and stop assembling blocks we would reject

CheckBlock() validated block size against MAX_SIZE, the serializer's generic
32 MB guard, not a block rule. MAX_BLOCK_SIZE did not exist in this tree at
all: 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. Upstream #111 (from #110 by mtg0001).

The cap is a tightening, so a node that does not upgrade is the permissive one.
Measured upstream over 5709 blocks: largest 34,290 bytes, median 216, so 1 MB
rejects nothing that exists.

Upstream shipped the validation half only. Both places that ASSEMBLE a block
still filled to MAX_SIZE/2 (16 MB), which was consistent against a 32 MB
validation cap and is inverted by a 1 MB one -- the node would solve a block
and then reject it itself. Fixed both:

  main.cpp  CreateNewBlock, the node's own miner
  rpc.cpp   RebuildJob, the POOL's job builder -- the one that matters here,
            because every miner on the pool would be working a template the
            network rejects, and this pool is ~90% of hashrate

Half the cap in both, preserving the original margin: each test runs against a
transaction that is already counted, so the loop can overshoot by one.

(cherry picked from commit a3be9f04903447fa2741aff4eb529fe45f916877)

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