Handle corrupt wallets gracefully.

Corrupt wallets used to cause a DB_RUNRECOVERY uncaught exception and a
crash. This commit does three things:

1) Runs a BDB verify early in the startup process, and if there is a
low-level problem with the database:
  + Moves the bad wallet.dat to wallet.timestamp.bak
  + Runs a 'salvage' operation to get key/value pairs, and
    writes them to a new wallet.dat
  + Continues with startup.

2) Much more tolerant of serialization errors. All errors in deserialization
are reported by tolerated EXCEPT for errors related to reading keypairs
or master key records-- those are reported and then shut down, so the user
can get help (or recover from a backup).

3) Adds a new -salvagewallet option, which:
 + Moves the wallet.dat to wallet.timestamp.bak
 + extracts ONLY keypairs and master keys into a new wallet.dat
 + soft-sets -rescan, to recreate transaction history

This was tested by randomly corrupting testnet wallets using a little
python script I wrote (https://gist.github.com/3812689)

src/db.h

@@ -50,6 +50,25 @@ public:
     ~CDBEnv();
     void MakeMock();
     bool IsMock() { return fMockDb; };
+
+    /*
+     * Verify that database file strFile is OK. If it is not,
+     * call the callback to try to recover.
+     * This must be called BEFORE strFile is opened.
+     * Returns true if strFile is OK.
+     */
+    enum VerifyResult { VERIFY_OK, RECOVER_OK, RECOVER_FAIL };
+    VerifyResult Verify(std::string strFile, bool (*recoverFunc)(CDBEnv& dbenv, std::string strFile));
+    /*
+     * Salvage data from a file that Verify says is bad.
+     * fAggressive sets the DB_AGGRESSIVE flag (see berkeley DB->verify() method documentation).
+     * Appends binary key/value pairs to vResult, returns true if successful.
+     * NOTE: reads the entire database into memory, so cannot be used
+     * for huge databases.
+     */
+    typedef std::pair<std::vector<unsigned char>, std::vector<unsigned char> > KeyValPair;
+    bool Salvage(std::string strFile, bool fAggressive, std::vector<KeyValPair>& vResult);
+
     bool Open(boost::filesystem::path pathEnv_);
     void Close();
     void Flush(bool fShutdown);
@@ -58,6 +77,7 @@ public:
     bool GetDetach() { return fDetachDB; }
 
     void CloseDb(const std::string& strFile);
+    bool RemoveDb(const std::string& strFile);
 
     DbTxn *TxnBegin(int flags=DB_TXN_WRITE_NOSYNC)
     {