Add optional transaction index to databases

By specifying -txindex when initializing the database, a txid-to-diskpos
index is maintained in the blktree database. This database is used to
help answering getrawtransaction() RPC queries, when enabled.

Changing the -txindex value requires a -reindex; the client will abort
at startup if the database and the specified -txindex mismatch.

src/txdb.h

@@ -43,6 +43,10 @@ public:
     bool WriteLastBlockFile(int nFile);
     bool WriteReindexing(bool fReindex);
     bool ReadReindexing(bool &fReindex);
+    bool ReadTxIndex(const uint256 &txid, CDiskTxPos &pos);
+    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();
 };