utils: Improve the EVR equality

It seems that with a newer version of SQLAlchemy I'm going through this
code when querying the DB for packages, even when not filtering on the
EVR.

And when no package is found by the query, SQLAlchemy uses this value
for the "other": <symbol 'NO_VALUE>

No idea why this is happening, but this additional check can't hurt
anyway.
sync: Don't go further if the package is blocked

If the package has been blocked since the last run, we are deleting it
from the DB.

As such, there is really no point trying to find its upstream, let's
skip directly to the next package.
sync: Improve log message
sync: Reorder some code

This makes more sense to me: first the two cases we ignore, then the two
cases we will continue acting upon.