Change ownership of pointers in Minimizer::Options.

This is a intermediate change to clean things up
in preparation for a broader refactoring of the SolverImpl.

Essentially we are replacing raw pointers in Minimizer::Options
with shared_ptr objects. For now this only makes things a bit
more complicated looking inside solver_impl.cc, but going
forward this will lead to considerable simplifications in
tracking ownership of various pointers.

Change-Id: I21db8fc6763c29b0d15e834d7c968a0f514042a0
9 files changed