Benchmark CBOR encoding/decoding using mainnet data

[fxamacker]

Closes #765
Updates #738

Description

Create benchmarks that use testdata/*.cbor.gz files.

Add initial 6 *.cbor.gz files using data from a mainnet snapshot.

• Filenames currently include v3 or v2 to indicate version (v4 is default but can be included in the name).
• The 16 character hex portion is a truncated SHA256 of the .cbor.gz file.

For safety:

• limit number of files processed (currently 100).
• use io.LimitReader to limit the max bytes uncompressed from each file (currently 16 MiB).

Caveats

The SHA256 isn't verified by the benchmark. We can do that or we can do it manually using sha256sum command and an extra file if desired.

I chose SHA256 because it's included in Go.

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For contributor use:

• Targeted PR against feature/storage-optimizations branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[turbolent]

Very nice!