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/main.cpp

@@ -3706,7 +3706,7 @@ CBlockIndex * InsertBlockIndex(uint256 hash)
 bool static LoadBlockIndexDB()
 {
     const CChainParams& chainparams = Params();
-    if (!pblocktree->LoadBlockIndexGuts())
+    if (!pblocktree->LoadBlockIndexGuts(InsertBlockIndex))
         return false;
 
     boost::this_thread::interruption_point();