chaincfg: Introduce v2 module.

[davecgh]

NOTE: This currently relies on an unreleased version of the chaincfg module (which is why it is failing Travis). That will need to be released before this can be merged. However, it is otherwise ready for review.

The chaincfg package currently has some less-than ideal APIs and behaviors that have been retained up to this point in order to avoid breaking existing caller code. However, now that versioned module support is available and in full use, these items can be addressed.

A quick overview of the issues with chaincfg this aims to address are:

• The block one 1 ledger is defined in terms of address strings, which forces consensus code to deal with standardness/policy code which it should not have to do (See chaincfg/blockchain: Change Block One Ledger To Require Exact Scripts #1659)
• It provides, and relies on, global registration which has side effects upon importing the package and can cause issues if multiple copies of the same package try to register themselves
• The current network parameter definitions are all package-level globals which have a few undesirable characteristics (See chaincfg: Convert Network Definitions To Functions #1667):
  • They are subject to mutation and thus can easily cause subtle and hard to find bugs
  • Many of the fields are defined in terms of others, but due to the struct being defined at the top level, it's not possible to reference other fields within it
  • They require a lot of unexported global variable state
  • It makes it difficult to introduce a base set of parameters that would likely help highlight exactly what the differences are between the networks
• It exports several constants which are not per-network constants and therefore do not belong in chaincfg
  • Further, because the constants are exported, they couldn't be changed without a major version bump, which would cause everything that relies on it to also bump majors.

Consequently, this introduces chaincfg/v2 which addresses the aforementioned items. A series of individual commits to make the review process easier. Each commit message more thoroughly describes its purpose, but primarily they consist of the following:

• Freeze version 1 of the chaincfg module
• Remove the root module chaincfg override so building the software will still produce binaries based on the v1 module until the v2 module is fully released
• Remove several constants which are not per-network and do not belong
• Minor consistency cleanup in order definition and tests
• Add tests for required unique fields in lieu of relying on registration to catch duplicates
• Remove all registration capabilities and associated functions
• Move code solely related to each network into the file for that network
• Use a concrete for the genesis hash in the Params struct
• Use scripts in block one token payouts instead of addresses
• Convert global network parameter defintions to functions
• Bump the module version to v2
• Update docs for the new module version

Finally, it should also be noted that this only introduces the new module and does not update anything to make use of it yet, so building the software will still produce binaries based on the v1 module.

---

For reference when moving to the new version of the module, the following is a list of changes to the public API:

Removed Functions:

• Register
• IsPubKeyAddrID
• IsPubKeyHashAddrID
• IsPKHEdwardsAddrID
• IsPKHSchnorrAddrID
• IsScriptHashAddrID
• HDPrivateKeyToPublicKeyID
• ParamsByNetAddrPrefix

Added Functions:

• MainNetParams
• TestNet3Params
• SimNetParams
• RegNetParams

Removed Constants:

• MainNetParams (Use MainNetParams() func instead)
• TestNet3Params (Use TestNet3Params() func instead)
• SimNetParams (Use SimNetParams() func instead)
• RegNetParams (Use RegNetParams() func instead)
• SigHashOptimization
• CheckForDuplicateHashes
• CPUMinerThreads
• ErrDuplicateNet
• ErrUnknownHDKeyID
• ErrDuplicateNetAddrPrefix
• ErrUnknownNetAddrPrefix

---

Closes #1659.
Closes #1667.