Add CurrentBlock in backends.BlockChainForCaller

[2dvorak]

Proposed changes

• CurrentHeader() may mismatch with CurrentBlock() (see Fix to resolve node loading failure after unexpected node termination #1766).
• So add CurrentBlock() in backends.BlockChainForCaller to replace CurrentHeader()
• Following interfaces or structs have direct/indirect dependency with backends.BlockChainForCaller so also added CurrentBlock() to them
  • reward.blockChain is used as backends.BlockChainForCaller
  • consensus.ChainReader is used as backends.BlockChainForCaller
  • blockchain.HeaderChain is used as consensus.ChainReader
    • Since HeaderChain does not have blocks, its implementation of CurrentBlock immediately panics

Types of changes

Please put an x in the boxes related to your change.

• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

• See this comment in Add BlockchainContractCaller #1828

Further comments

None