blockchain: Introduce v2 module.

[davecgh]

This requires #1820.

NOTE: This PR currently has a replacement for the blockchain module that will need to be updated prior to merging this PR once said module is merged and tagged.

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In order to make use of chaincfg/v2, dcrutil/v2, blockchain/stake/v2, database/v2 and txscript/v2, a new major version of blockchain module is required because blockchain/v1 accepts and returns types from all of them in its API.

Since a new major version is required, this also takes the opportunity handle a couple of other items that require a major version bump:

• Remove the functions, types, return values, and parameters that were previously deprecated
• Accept a SubsidyCache to use from the caller instead of creating a local one the caller must later query

Consequently, this introduces blockchain/v2 which addresses the aforementioned items. A series of individual commits is provided 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 blockchain module use
• Remove the root module blockchain replace so building the software will still produce binaries based on the v1 module until the v2 module is fully released
• Use a local constant for the old txscript.CheckForDuplicateHashes optimization toggle
• Remove the following deprecated items:
  • DisableLog from both blockchain and indexers packages
  • BlockOneCoinbasePaysTokens
  • IsCoinBase
  • IsCoinBaseTx
  • BuildMerkleTreeStore
  • BuildMsgTxMerkleTreeStore
  • SubsidyCache
  • NewSubsidyCache
  • CalcBlockWorkSubsidy
  • CalcStakeVoteSubsidy
  • CalcBlockTaxSubsidy
  • CoinbasePaysTax
  • CalculateAddedSubsidy
  • BestPrevHash
  • TotalSubsidy
  • FetchSubsidyCache
  • HashToBig
  • CompactToBig
  • BigToCompact
  • CalcWork
  • CheckProofOfWork
• Update to use the following deps:
  • blockchain/stake/v2@v2.0.1
  • chaincfg/v2@v2.2.0
  • database/v2@v2.0.0
  • dcrutil/v2@v2.0.0
  • txscript/v2@v2.0.0
• 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.

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The following table provides a mapping of the removed items to their equivalents to aid consumers in upgrading:

Removed Item	New Alternative
DisableLog	UseLogger(slog.Disabled)
BlockOneCoinbasePaysTokens	no alternative, internal only
IsCoinBase	standalone.IsCoinBaseTx
IsCoinBaseTx	standalone.IsCoinBaseTx
BuildMerkleTreeStore	standalone.CalcTxTreeMerkleRoot
BuildMsgTxMerkleTreeStore	standalone.CalcTxTreeMerkleRoot
SubsidyCache	standalone.SubsidyCache
NewSubsidyCache	standalone.NewSubsidyCache
CalcBlockWorkSubsidy	standalone.SubsidyCache.CalcWorkSubsidy
CalcStakeVoteSubsidy	standalone.SubsidyCache.CalcStakeVoteSubsidy
CalcBlockTaxSubsidy	standalone.SubsidyCache.CalcTreasurySubsidy
CoinbasePaysTax	no alternative, internal only
CalculateAddedSubsidy	no alternative, internal only
BestPrevHash	BestSnapshot.PrevHash
TotalSubsidy	BestSnapshot.TotalSubsidy
FetchSubsidyCache	standalone.NewSubsidyCache passed to chain via Config
HashToBig	standalone.HashToBig
CompactToBig	standalone.CompactToBig
BigToCompact	standalone.BigToCompact
CalcWork	standalone.CalcWork
CheckProofOfWork	standalone.CheckProofOfWork

[davecgh]

This shows as closed, however, it was merged.