`LinearProgram` symbol serialization doesn't include feasibility information

[arcondello]

Currently the LinearProgram symbol always saves the state whether it's feasible or not

dwave-optimization/dwave/optimization/symbols/lp.pyx

Lines 280 to 282 in 6a655f6

	# we save the state regardless of whether it is feasible or not
	# In the future we could choose to ignore infeasible states.
	states[state_index, :] = self.state(state_index)

however, this creates an issue. The state contents are undefined and may in fact encode a feasible solution to the LP, or may contain nans and infs.

Proposed Solution
Add another feasibility array to the serialized state. This creates 6 cases (because we need to support unspecified i.e., serialized with <=0.7.0)

	feasible	infeasible	unspecified
contains nan/inf(s)	no state1	generate a new state, mark infeas2	no state
no nan/inf(s)	good state	use state, mark infeas	use state

1. This is a bit unintuitive, but we need a way to specify "no state" in the new schema. We could add yet another array but that feels like overkill.
2. The generated state might itself be feasible, but the docs say that the state of an LP symbol propagating infeasible is undefined.