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/walletdb.h

@@ -17,6 +17,7 @@ enum DBErrors
 {
     DB_LOAD_OK,
     DB_CORRUPT,
+    DB_NONCRITICAL_ERROR,
     DB_TOO_NEW,
     DB_LOAD_FAIL,
     DB_NEED_REWRITE
@@ -153,8 +154,10 @@ public:
     int64 GetAccountCreditDebit(const std::string& strAccount);
     void ListAccountCreditDebit(const std::string& strAccount, std::list<CAccountingEntry>& acentries);
 
-    int ReorderTransactions(CWallet*);
-    int LoadWallet(CWallet* pwallet);
+    DBErrors ReorderTransactions(CWallet*);
+    DBErrors LoadWallet(CWallet* pwallet);
+    static bool Recover(CDBEnv& dbenv, std::string filename, bool fOnlyKeys);
+    static bool Recover(CDBEnv& dbenv, std::string filename);
 };
 
 #endif // BITCOIN_WALLETDB_H