Move ::nMinimumChainWork into ChainstateManager

This changes the minimum chain work for the bitcoin-chainstate
executable. Previously it was uint256{}, now it is the chain's default
minimum chain work.

src/validation.h

@@ -99,9 +99,6 @@ extern bool g_parallel_script_checks;
 extern bool fCheckBlockIndex;
 extern bool fCheckpointsEnabled;
 
-/** Minimum work we will assume exists on some valid chain. */
-extern arith_uint256 nMinimumChainWork;
-
 /** Documentation for argument 'checklevel'. */
 extern const std::vector<std::string> CHECKLEVEL_DOC;
 
@@ -865,6 +862,7 @@ public:
 
     const CChainParams& GetParams() const { return m_options.chainparams; }
     const Consensus::Params& GetConsensus() const { return m_options.chainparams.GetConsensus(); }
+    const arith_uint256& MinimumChainWork() const { return *Assert(m_options.minimum_chain_work); }
     const uint256& AssumedValidBlock() const { return *Assert(m_options.assumed_valid_block); }
 
     /**