Fix double serialize in CheckpointTrie._formatNode()

[holgerd77]

This PR fixes the double-execution of Node serialization in CheckpointTrie._formatNode() as well as moves a unified _formatNode() to BaseTrie (the added somewhat dummy isCheckpoint() getter there avoids a TypeScript error) to remove redundancy.

This gives a significant performance increase when running the checkpointing benchmarks:

Run on PR:

$ ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts
benchmarks/checkpointing.ts | average execution time: 0s 148.515ms
benchmarks/checkpointing.ts | average execution time: 0s 146.228ms
benchmarks/checkpointing.ts | average execution time: 0s 150.828ms
benchmarks/checkpointing.ts | average execution time: 0s 146.222ms
benchmarks/checkpointing.ts | average execution time: 0s 152.767ms

$ ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts
benchmarks/checkpointing.ts | average execution time: 0s 146.98ms
benchmarks/checkpointing.ts | average execution time: 0s 144.115ms
benchmarks/checkpointing.ts | average execution time: 0s 156.19ms
benchmarks/checkpointing.ts | average execution time: 0s 148.622ms
benchmarks/checkpointing.ts | average execution time: 0s 149.105ms

Run on master:

$ ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts
benchmarks/checkpointing.ts | average execution time: 0s 172.482ms
benchmarks/checkpointing.ts | average execution time: 0s 176.066ms
benchmarks/checkpointing.ts | average execution time: 0s 181.225ms
benchmarks/checkpointing.ts | average execution time: 0s 180.5ms
benchmarks/checkpointing.ts | average execution time: 0s 172.593ms

$ ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts && ts-node benchmarks/checkpointing.ts
benchmarks/checkpointing.ts | average execution time: 0s 183.217ms
benchmarks/checkpointing.ts | average execution time: 0s 173.598ms
benchmarks/checkpointing.ts | average execution time: 0s 174.573ms
benchmarks/checkpointing.ts | average execution time: 0s 172.899ms
benchmarks/checkpointing.ts | average execution time: 0s 172.908ms

[coveralls]

Coverage increased (+0.6%) to 95.29% when pulling 40b02ee on fix-double-serialize-in-checkpoint-trie-format-node into 7add2b2 on master.

[jochem-brouwer]

I was a bit confused here but this is the "unpacked" node.hash() function where we just hash this raw rlpNode data.

[jochem-brouwer]

Looks good, great find! 😄