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)