This introduces a 'minimum chain work' chainparam which is intended
to be the known amount of work in the chain for the network at the
time of software release. If you don't have this much work, you're
not yet caught up.
This is used instead of the count of blocks test from checkpoints.
This criteria is trivial to keep updated as there is no element of
subjectivity, trust, or position dependence to it. It is also a more
reliable metric of sync status than a block count.
GetTotalBlocksEstimate is no longer used and it was the only thing
the checkpoint tests were testing.
Since checkpoints are on their way out it makes more sense to remove
the test file than to cook up a new pointless test.