Tweak blkindex.dat further.

This patch optimises transaction verification and the blkindex.dat file a bit more.  Tx verification now takes even less disk access as before, and the data stored in blkindex.dat about each transaction is removed to the bare minimum (for now - only the tx disk position and no longer any additional information).

On the first run with this patch, blkindex.dat will be upgraded/rewritten (as already some times before) - this takes quite some time, disk access and memory (around 2 GiB of RAM during upgrade).  It seems to work fine so far for me, but I haven't yet tested a full sync on mainnet (testnet works).  Also, it changes some parts of mining code, so it would be good to have it tested carefully by some miners before they upgrade fully.  (But again, mining on testnet seems to work fine.)