When creating new blocks, sort 'paid' area by fee-per-kb

Modify CreateNewBlock so that instead of processing all transactions
in priority order, process the first 27K of transactions in
priority order and then process the rest in fee-per-kilobyte
order.

This is the first, minimal step towards better a better fee-handling
system for both miners and end-users; this patch should be easy
to backport to the old versions of Bitcoin, and accomplishes the
most important goal-- allow users to "buy their way in" to blocks
using transaction fees.