Change the default mode of CachingOptimizer

[odow]

Another way of fixing #1249.

The default of a CachingOptimizer is to initialize in ATTACHED_OPTIMIZER mode. This has two consequences:

• Solvers that do not support incremental building will throw an error at the first incremental call and reset the optimizer.
• Solvers that do support incremental building will build incrementally, even if they have a fast copy_to available.

It seems like a much more sensible default is to initialize in EMPTY_OPTIMIZER mode.

• Solvers that do not support incremental building no longer need to catch an error
• Solvers that do support incremental building will use the fast copy_to if available.

This is breaking (see the test change), so we may want to hold off until MOI 0.10, but it does need doing.

[mlubin]

Makes sense to me, pretty clean fix. Agreed it should be for MOI 0.10.

[blegat]

Looks good. I don't think it's breaking. We can release this in a patch release. It only affect the state in AUTOMATIC mode anyway