Return initial difficulty for when the previous block is Genesis

[alfiedotwtf]

More info in #1002

[alfiedotwtf]

CI failed on Ubuntu, but I don't capture the error. I'll update all the blockprod tests so we can see what's happening when they fail.

[alfiedotwtf]

The failed test in CI was cancel_received() in blockprod/src/detail/test.rs. This test creates a new Genesis with an initial difficulty of zero...

The 57ee633 commit was supposed to return the chain config's current PoW limit i.e zero, however after following the call stack, it was never using the overridden value, as seen in get_proof_of_work_config() from commit e2b9d75.

These last few commits creates a PoWChainConfig builder which allows the caller to override chain settings -as is done now in get_proof_of_work_config() to override the difficulty by looking at NetUpgrades (if available) rather than straight to the default value.