Add a name field to NetworkParameters to be used at the end of the cache dir path

[arya2]

Motivation

We want to ensure that the cached state matches the consensus rules for a network when adding Regtest or custom testnets.

Some users may want to use part of the Testnet chain for custom testnets, this could be done with the CopyState command:
zebrad copy-state --max-source-height 1200000 --target-config-path path/to/custom-testnet-config. This won't work for Regtest because the db format is different.

Design

• Add the network name to the cache_dir path to store the Regtest state separately from the Testnet state. The chainstate is stored in a regtest directory in zcashd.
• Panic with a "reserved network name" message if the value is set to "Mainnet", "Testnet", or "Regtest" for a configured testnet