Testnet 5 launch

Hello world,

I propose mining testnet 5's genesis block at the end of the week, aligned with the next mainnet retarget at block 975,744.

If people want to point hashrate at bootstrapping testnet5, I've got some docs on the process available at:

  https://gist.github.com/ajtowns/ebb148819c6028387271e6f0dce5e1da

and the 0%-fee solo-mining pool to point to is available at:

  https://lapis.erisian.com.au/chain.html

It will keep running tests at lower difficulty for the rest of the week until shortly before block 975,744 is mined.

Couple of extra bits of info:

This setup involves a couple of tweaks to the genesis block structure, documented in https://github.com/bitcoin/bips/pull/2347 . These are to make pooled mining of the genesis block easier (providing extranonce space and expicitly allowing nVersion rolling), and also to make it a bit closer to previous genesis blocks (having the commitment be text in the pszTimestamp field rather than the output, and making the output an unspendable OP_RETURN rather than a potentially-post-quantum spendable x-point).

When bootstrapping a new chain, the ambiguity in how BIP 34 block height's are encoded tends to bite: bitcoin core expects blocks 1 to 16 to use OP_1 to OP_16 in the coinbase, but a lot of mining software expects to see a one-byte push, so 0x0101 to 0x0110, so if you see weird behaviour at low height blocks, this might be a cause. References:

  * https://github.com/bitcoin/bips/pull/2342
  * https://github.com/bitaxeorg/ESP-Miner/pull/2089

Cheers, aj