Rework and Improve addblock utility.

The addblock utility was originally written as a quick debug tool during
initial development to populate blocks into the database.  However, now
that it has been designated as the standard way to import bootstrap.dat
(and indeed block data files in general), it was lacking a few features
such as properly checking against the chain rules and known good
checkpoints.

This commit reworks and improves the utility in several ways:

- Imported blocks are now checked against the chain rules including
  checkpoints to ensure they match the known good chain
- The utility now properly shuts down after processing all blocks
- Attempting to import orphan blocks (blocks which build off a block you
  don't yet have in the database) returns an error
- Blocks that are already known are now skipped instead of causing an
  error which means you can stop and restart the import mid-way through
  without issues or start it after you've already downloaded a
  portion of the chain
- The block height is no longer assumed to start at 0 which means input
  files that start later in the chain will work properly so long as you
  already have the chain at least up to the point of the block just before
  the first one in the input file
- Improved error handling and reporting
- How often the progress display is shown is now configurable
- Statistics about how many blocks were processed, imported, and already
  known are now displayed after the input file has been fully processed

This resolves comments made in #60.