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.cpp

@@ -5,10 +5,8 @@
 
 #include "txdb.h"
 
-#include "chain.h"
 #include "chainparams.h"
 #include "hash.h"
-#include "main.h"
 #include "pow.h"
 #include "uint256.h"
 
@@ -177,7 +175,7 @@ bool CBlockTreeDB::ReadFlag(const std::string &name, bool &fValue) {
     return true;
 }
 
-bool CBlockTreeDB::LoadBlockIndexGuts()
+bool CBlockTreeDB::LoadBlockIndexGuts(boost::function<CBlockIndex*(const uint256&)> insertBlockIndex)
 {
     boost::scoped_ptr<CDBIterator> pcursor(NewIterator());
 
@@ -191,8 +189,8 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
             CDiskBlockIndex diskindex;
             if (pcursor->GetValue(diskindex)) {
                 // Construct block index object
-                CBlockIndex* pindexNew = InsertBlockIndex(diskindex.GetBlockHash());
-                pindexNew->pprev          = InsertBlockIndex(diskindex.hashPrev);
+                CBlockIndex* pindexNew = insertBlockIndex(diskindex.GetBlockHash());
+                pindexNew->pprev          = insertBlockIndex(diskindex.hashPrev);
                 pindexNew->nHeight        = diskindex.nHeight;
                 pindexNew->nFile          = diskindex.nFile;
                 pindexNew->nDataPos       = diskindex.nDataPos;