DB 4.7 -> 4.8 upgrade for 0.4.0

[Matt Corallo]

I was thinking, when we move to encrypted wallet in 0.4.0, we might as well upgrade the BDB we are releasing our builds with to 4.8 from 4.7.  The debian packages already link against 4.8, as do the Mac releases (assuming build-osx.txt is right).  The actual databases formats are identical, but the log format changes (not 100% sure here, but that is what I've been told by several people).  That would mean it should never be an issue for end users, but if they start copying wallets around from OSX to Win with the logs, they might see DB errors.  Comments, concerns, questions?