#4047 Fail to compile with Protobuf2

[mastash3ff]

See issue #4047 for more details on the problems encountered and fix.

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[mastash3ff]

I signed it!

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CLAs look good, thanks!

[pkulzc]

This "reserved" is required so future user would not accidentally reuse a deprecated field number.
Try using protobuf3 as described in the updated installation doc.

[pkulzc]

I'm closing this PR, but feel free to reopen it if you have any questions, thanks!