change our on-disk format slightly: we now store block size in the header, this
is important for getting progress bars that are more granular, and also
if you exit Armory and restart, it brings the progress bars to where they left off.

I also renamed StoredHeader::unserialize to setHeaderData, to make it more
clear that it doesn't unserialize, and leaves a number of variables unset.
don't make pytest depend on building gtest, so that make pytest doesn't
make me rebuild gtest which takes a while
make progress bars more granular. If you exit Armory while scanning and restart,
progress bars go back to where they were before.

Create a "Organizing chain" progress state, so things look snappier. Also create
a "load old block headers" progress state.

This patch depends on the new LMDB format.