[Solver Framework] Refactor MaxSat Backends

[CharlesZ-Chen]

Currently there were a lot code duplication between MaxSatBackEnd and LingelingBackEnd.

Should better to refactor this:

• Abstract a common Backend named MaxSatBackEnd, that described the general solving logic for MaxSat-like solvers

• Have two concrete sub-classes, Sat4j and Lingeling that adapt the concrete solvers.