Resolve the checkpoints <-> validation CD.

This commit resolves the checkpoints -> validation -> checkpoints
cirular dependency by moving
`CheckPoints::GetLastCheckpoint(const CCheckpointData& data)` from
`checkpoints.cpp` to `validation.cpp`.

src/validation.cpp

@@ -8,7 +8,6 @@
 #include <arith_uint256.h>
 #include <chain.h>
 #include <chainparams.h>
-#include <checkpoints.h>
 #include <checkqueue.h>
 #include <consensus/consensus.h>
 #include <consensus/merkle.h>
@@ -35,6 +34,7 @@
 #include <txdb.h>
 #include <txmempool.h>
 #include <ui_interface.h>
+#include <uint256.h>
 #include <undo.h>
 #include <util/moneystr.h>
 #include <util/strencodings.h>
@@ -3196,6 +3196,22 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
     return commitment;
 }
 
+//! Returns last CBlockIndex* that is a checkpoint
+static CBlockIndex* GetLastCheckpoint(const CCheckpointData& data)
+{
+    const MapCheckpoints& checkpoints = data.mapCheckpoints;
+
+    for (const MapCheckpoints::value_type& i : reverse_iterate(checkpoints))
+    {
+        const uint256& hash = i.second;
+        CBlockIndex* pindex = LookupBlockIndex(hash);
+        if (pindex) {
+            return pindex;
+        }
+    }
+    return nullptr;
+}
+
 /** Context-dependent validity checks.
  *  By "context", we mean only the previous block headers, but not the UTXO
  *  set; UTXO-related validity checks are done in ConnectBlock().
@@ -3220,7 +3236,7 @@ static bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationSta
         // Don't accept any forks from the main chain prior to last checkpoint.
         // GetLastCheckpoint finds the last checkpoint in MapCheckpoints that's in our
         // MapBlockIndex.
-        CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint(params.Checkpoints());
+        CBlockIndex* pcheckpoint = GetLastCheckpoint(params.Checkpoints());
         if (pcheckpoint && nHeight < pcheckpoint->nHeight)
             return state.DoS(100, error("%s: forked chain older than last checkpoint (height %d)", __func__, nHeight), REJECT_CHECKPOINT, "bad-fork-prior-to-checkpoint");
     }