Faster initial block download (5x faster)

By making some adjustments to the database settings, I was able to make the initial block download about 5 times faster.  It downloads in about 30 minutes.

The database default had it writing each block to disk synchronously, which is not necessary.  I changed the settings to let it cache the changes in memory and write them out in a batch.  Blocks are still written transactionally, so either the complete change occurs or none of it does, in either case the data is left in a valid state.

I only enabled this change during the initial block download.  When you come within 2000 blocks of the latest block, these changes turn off and it slows down to the old way.

I built a test build if you'd like to start using it:

These binaries also include Gavin Andresen's JSON-RPC HTTP authentication feature and the other important security improvements from 0.3.2.

I've been running a test over the last 24 hours that kills and restarts it randomly every 2-60 seconds (poor thing) while it's trying to do an initial block download and it's been fine.

There are no changes to the way it handles wallet.dat.  This change is only for blk*.dat and the non-critical addr.dat.  You can always delete blk*.dat if it gets screwed up and let it re-download.