Reject blocks that fork before previous checkpoint.

This commit adds an additional check to the block acceptance rules which
prevents new blocks that fork the main chain before the previous known
good checkpoint.  This prevents storage of new, otherwise valid, blocks
from building off of old blocks which are likely at a much easier
difficulty and therefore could be used to waste cache and disk space.

Note this is slightly different than the other existing check which
prevents blocks with a timestamp before the timestamp of the latest known
good checkpoint since that check effectively prevents old side chain
blocks that already existed (per the claimed timestamp).

ok drahn@
Rename findLatestKnownCheckpoint.

The name findLatestKnownCheckpoint is confusing and doesn't really convey
the fact the purpose of the function is to the find the checkpoint prior
to the current known block.

Therefore, rename the function to findPreviousCheckpoint for clarity.

Also, update some comments to follow suit.