Downgrading from compressed keys #856

When starting an older bitcoin version after using 0.6:

• if the wallet is not encrypted, it will use the non-compressed variants of the keys, eventually diverging
• if the wallet is encrypted, decrypting it will cause a "wrong password" warning

Solution: write minversion in the wallet

Dont forget to write a CCorruptAddress every time minversion is written to to ensure clients that dont know about minversion ignore die when trying to load the wallet.

#864 merged