[C++] Reorganize how num_multistarts (and other multistart info) is passed around

[suntzu86]

Currently GradientDescentParameters and NewtonParameters lets you specify a "num_multistarts" field. But for 'dumb' search, this is specified as a plain int.

This doesn't make a lot of sense.

Remove num_multistarts from ALL OptimizerParameter structs. num_multistarts doesn't configure the individual optimizer, it just controls how many times we call that optimizer. This will match how the parameter structs look in the Python modules.

Then num_multistarts can live in a parameter struct specifically for multistart optimization. At first this will just hold num_multistarts, but later as we consider more complex multistart heuristics (GH-165), the multistart parameter struct can be expanded.