Merge #13204: Faster sigcache nonce

152e8baf08c7379e5cc09f90863e6309bdd4866c Use salted hasher instead of nonce in sigcache (Jeremy Rubin)
5495fa585007b40b2e9285c23be275de71708af8 Add Hash Padding Microbenchmarks (Jeremy Rubin)

Pull request description:

  This PR replaces nonces in two places with pre-salted hashers.

  The nonce is chosen to be 64 bytes long so that it forces the SHA256 hasher to process the chunk. This leaves the next 64 (or 56 depending if final chunk) open for data. In the case of the script execution cache, this does not make a big performance improvement because the nonce was already properly padded to fit into one buffer, but does make the code a little simpler. In the case of the sig cache, this should reduce the hashing overhead slightly because we are less likely to need an additional processing step.

  I haven't benchmarked this, but back of the envelope it should reduce the hashing by one buffer for all combinations except compressed public keys with compact signatures.

ACKs for top commit:
  ryanofsky:
    Code review ACK 152e8baf08c7379e5cc09f90863e6309bdd4866c. No code changes, just rebase since last review and expanded commit message

Tree-SHA512: b133e902fd595cfe3b54ad8814b823f4d132cb2c358c89158842ae27daee56ab5f70cde2585078deb46f77a6e7b35b4cc6bba47b65302b7befc2cff254bad93d

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