Add a new checkpoint at block 279,000

From a cursory glance, it looks like block 279,000 meets the criteria for
a checkpoint block, and it's also what the bootstrap.dat torrent is at. This closes #3530.
Update linearize.py defaults to block 279,000
Merge pull request #3551

6873cfb Update linearize.py defaults to block 279,000 (Micha)
e7906d1 Add a new checkpoint at block 279,000 (Micha)