grin: Investigate reason for high number of **rejected** mined shares

A fast miner (1080 ti GPU) sees a high number of both rejected and stale shares. ex: “974 accepted, 44 rejected and 25 stales”

A slow miner (1-core on an i7 cpu) sees far fewer stale shares but still sees many shares rejected due to invalid POW. ex: “109 accepted, 16 rejected, 0 stale”

We need to understand why the shares are rejected:

• does the miner return invalid pow?
• does the data get corrupted in transit?
• is there some issue with the validation code?
• something else?

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The grin.log messages associated with rejected shares:

Jun 30 00:25:29.786 DEBG pipe: process_block b67f0bcb at 126038 with 0 inputs, 1 outputs, 1 kernels
Jun 30 00:25:29.787 ERRO pipe: validate_header failed for cuckoo shift size 30
Jun 30 00:25:29.787 INFO Rejected block b67f0bcb at 126038: InvalidPow
Jun 30 00:25:29.789 ERRO (Server ID: StratumServer) Failed to validate solution at height 126038: InvalidPow