NOMERGE: [0.17] Signet implementation

[jtimon]

A rebase of kallewoof/bitcoin#5 on top of #432 taking the opportunity to simplify its changes around chainparams code and adding a test that also acts as a script to generate a private key in wallet importable format and its corresponding p2pk blockscript, which should be useful for people wanting to generate their own signets.
This represent a hardfork from that implementation because the genesis block now includes the name of the chain too, like all other custom chains, and the bits have to be low enough so that tests keep working with regtest-like pow.

Example to generate chain arguments:

python3 ./test/functional/test_runner.py feature_signet --config_path=/home/user/signet

That will generate:

/home/user/signet/signet_blockscript.txt
/home/user/signet/wif.txt

Dependencies:

• [0.17] Test: Fix example_test.py [0.17] Test: Fix example_test.py #434
• [0.17] QA: Use resgtest2 chain instead of regtest for rpc tests [0.17] QA: Use resgtest2 chain instead of regtest for rpc tests #432
• [0.17] Chainparams: Introduce consensus.genesis_style for compatibility [0.17] Chainparams: Introduce consensus.genesis_style for compatibility #436

[instagibbs]

this needs +$min_time

[instagibbs]

We're going to work directly on a branch, rather than a PR on top of elements-0.17.

[jtimon]

You're going to need at least part of it here too, assuming #414 is merged and forward ported. putting it all facilitates testing since you wouldn't need a separated binary to test the fedpeg to a parent signet anymore.
But your call.

[instagibbs]

Not going to delete the branch to be clear. I made a copy to the repo: https://github.com/ElementsProject/elements/tree/e17-signet

IIRC I was getting conflicts as the base branch was moving along with features, and for now we just wanted a 0.17 Core branch to work on.