Break circular dependency main ↔ txdb

Break the circular dependency between main and txdb by:

- Moving `CBlockFileInfo` from `main.h` to `chain.h`. I think this makes
  sense, as the other block-file stuff is there too.

- Moving `CDiskTxPos` from `main.h` to `txdb.h`. This type seems
  specific to txdb.

- Pass a functor `insertBlockIndex` to `LoadBlockIndexGuts`. This leaves
  it up to the caller how to insert block indices.

src/chain.h

@@ -15,6 +15,60 @@
 
 #include <vector>
 
+class CBlockFileInfo
+{
+public:
+    unsigned int nBlocks;      //!< number of blocks stored in file
+    unsigned int nSize;        //!< number of used bytes of block file
+    unsigned int nUndoSize;    //!< number of used bytes in the undo file
+    unsigned int nHeightFirst; //!< lowest height of block in file
+    unsigned int nHeightLast;  //!< highest height of block in file
+    uint64_t nTimeFirst;       //!< earliest time of block in file
+    uint64_t nTimeLast;        //!< latest time of block in file
+
+    ADD_SERIALIZE_METHODS;
+
+    template <typename Stream, typename Operation>
+    inline void SerializationOp(Stream& s, Operation ser_action, int nType, int nVersion) {
+        READWRITE(VARINT(nBlocks));
+        READWRITE(VARINT(nSize));
+        READWRITE(VARINT(nUndoSize));
+        READWRITE(VARINT(nHeightFirst));
+        READWRITE(VARINT(nHeightLast));
+        READWRITE(VARINT(nTimeFirst));
+        READWRITE(VARINT(nTimeLast));
+    }
+
+     void SetNull() {
+         nBlocks = 0;
+         nSize = 0;
+         nUndoSize = 0;
+         nHeightFirst = 0;
+         nHeightLast = 0;
+         nTimeFirst = 0;
+         nTimeLast = 0;
+     }
+
+     CBlockFileInfo() {
+         SetNull();
+     }
+
+     std::string ToString() const;
+
+     /** update statistics (does not update nSize) */
+     void AddBlock(unsigned int nHeightIn, uint64_t nTimeIn) {
+         if (nBlocks==0 || nHeightFirst > nHeightIn)
+             nHeightFirst = nHeightIn;
+         if (nBlocks==0 || nTimeFirst > nTimeIn)
+             nTimeFirst = nTimeIn;
+         nBlocks++;
+         if (nHeightIn > nHeightLast)
+             nHeightLast = nHeightIn;
+         if (nTimeIn > nTimeLast)
+             nTimeLast = nTimeIn;
+     }
+};
+
 struct CDiskBlockPos
 {
     int nFile;