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/txdb.h

@@ -8,15 +8,17 @@
 
 #include "coins.h"
 #include "dbwrapper.h"
+#include "chain.h"
 
 #include <map>
 #include <string>
 #include <utility>
 #include <vector>
 
-class CBlockFileInfo;
+#include <boost/function.hpp>
+
 class CBlockIndex;
-struct CDiskTxPos;
+class CCoinsViewDBCursor;
 class uint256;
 
 //! -dbcache default (MiB)
@@ -26,7 +28,30 @@ static const int64_t nMaxDbCache = sizeof(void*) > 4 ? 16384 : 1024;
 //! min. -dbcache in (MiB)
 static const int64_t nMinDbCache = 4;
 
-class CCoinsViewDBCursor;
+struct CDiskTxPos : public CDiskBlockPos
+{
+    unsigned int nTxOffset; // after header
+
+    ADD_SERIALIZE_METHODS;
+
+    template <typename Stream, typename Operation>
+    inline void SerializationOp(Stream& s, Operation ser_action, int nType, int nVersion) {
+        READWRITE(*(CDiskBlockPos*)this);
+        READWRITE(VARINT(nTxOffset));
+    }
+
+    CDiskTxPos(const CDiskBlockPos &blockIn, unsigned int nTxOffsetIn) : CDiskBlockPos(blockIn.nFile, blockIn.nPos), nTxOffset(nTxOffsetIn) {
+    }
+
+    CDiskTxPos() {
+        SetNull();
+    }
+
+    void SetNull() {
+        CDiskBlockPos::SetNull();
+        nTxOffset = 0;
+    }
+};
 
 /** CCoinsView backed by the coin database (chainstate/) */
 class CCoinsViewDB : public CCoinsView
@@ -83,7 +108,7 @@ public:
     bool WriteTxIndex(const std::vector<std::pair<uint256, CDiskTxPos> > &list);
     bool WriteFlag(const std::string &name, bool fValue);
     bool ReadFlag(const std::string &name, bool &fValue);
-    bool LoadBlockIndexGuts();
+    bool LoadBlockIndexGuts(boost::function<CBlockIndex*(const uint256&)> insertBlockIndex);
 };
 
 #endif // BITCOIN_TXDB_H