_dot_multiply uses evaluate, therefore breaks fix!/free!

[ericphanson]

• If we use evaluate in a reformulation, then we are locking in the currently-fix!ed value to the expression tree, and later updates to it (or free!s won't be respected. This is the Sequentially solve a quadratic programming problem #585 issue.
• If we use evaluate during conic_form!, that's fine since that gets called every solve. (Though it will have to be figured out if we do Making Convex.jl DPP compliant #383).

One way we can fix this is by adding a DotMultiply atom so we can do the reformulation at conic_form! time instead of expression-building time, and therefore get the latest value. We could add a QuadForm atom also, to support fix!'d variables rather than erroring.

I couldn't find any other usages left. It would be nice to figure out a way to add to our atom tests that evaluate wasn't called, but I don't know a reasonable way to do that. (What I can think of is splitting recursive evaluate from non-recursive, then adding a global or ScopedValue based check to the non-recursive one to error depending on the context, but that isn't very nice).