Fix loading blocks from old-type checkpoints

[bzawisto]

Adding new fields to BlockHeader broke bincode serialization.

[github-actions]

Bencher Report

Branch	checkpoint-loading-fix
Testbed	self-hosted

Click to view all benchmark results

Benchmark	Latency	Benchmark Result milliseconds (ms) (Result Δ%)	Upper Boundary milliseconds (ms) (Limit %)
full-blocks-erc20-transfers/full-blocks-erc20-transfers	📈 view plot 🚷 view threshold	601.03 ms (-5.61%) Baseline: 636.73 ms	746.97 ms (80.46%)
full-blocks-evm-transfers/full-blocks-evm-transfers	📈 view plot 🚷 view threshold	882.92 ms (-1.65%) Baseline: 897.73 ms	1,090.79 ms (80.94%)
full-blocks-giant-deploy/full-blocks-giant-deploy	📈 view plot 🚷 view threshold	5,440.30 ms (+6.79%) Baseline: 5,094.36 ms	5,484.21 ms (99.20%)
full-blocks-scilla-add/full-blocks-scilla-add	📈 view plot 🚷 view threshold	1,037.20 ms (+2.80%) Baseline: 1,008.92 ms	1,050.02 ms (98.78%)
full-blocks-scilla-load/full-blocks-scilla-load	📈 view plot 🚷 view threshold	5,483.40 ms (+1.96%) Baseline: 5,377.74 ms	5,970.76 ms (91.84%)
full-blocks-scilla-store/full-blocks-scilla-store	📈 view plot 🚷 view threshold	4,432.60 ms (+3.35%) Baseline: 4,288.94 ms	4,783.12 ms (92.67%)
full-blocks-zil-transfers/full-blocks-zil-transfers	📈 view plot 🚷 view threshold	1,215.50 ms (+27.16%) Baseline: 955.88 ms	1,245.80 ms (97.57%)
process-empty/process-empty	📈 view plot 🚷 view threshold	13.51 ms (+27.89%) Baseline: 10.56 ms	13.83 ms (97.70%)

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