Decide what to record in solvers

[chaozg]

          I think this is a nice solution. We might want to create an issue in the future about passing everything in `solution` dictionary except `x` to info, because the dictionary content might differ from solver to solver. Something like 

sol = solution['x']
solution.pop('x')
info = solution

without the need to access specific keys

approved :)

Originally posted by @amal-ghamdi in #598 (review)