Don't parse '@>=' ranges by default

They are still used internally by the constraint solver (to implement
update --breaking) but cannot be externally specified.

Also, stop supporting "@none", whatever that was.
First try locking all previousSolution versions.

This is how some of us thought previousSolution already worked.
fix --slow report-stats test (broken by 66f21a6)
fix report-stats test from checkout

You'd think that when making a commit with a name like 'fix
reports-stats test from release', this "glasser" fellow would consider
checking to see if he broke the other use case, but that's apparently
too much to ask.