Add a debug_disable_pow_check field to Network

[arya2]

Motivation

We want to use this field to disable the equihash and target difficulty checks when validating blocks for Regtest.

[arya2]

We want to use the Regtest PoW parameters instead to preserve test coverage of consensus rules on Regtest.

The equihash parameters for Regtest are different from and easier than those on Mainnet/Testnet. We could add these parameters and the target difficulty limit as fields on NetworkParameters.

We may need to make changes to the internal-miner in Zebra and to the equihash solver in librustzcash to use the different parameters for Regtest. The equihash parameters we need should be in chainparams.cpp.

[arya2]

No harm in adding this alongside the Regtest equihash parameters or to be used until the internal miner is restored.