Minor refactoring of subsolver initialization (#4731)

Minor refactoring of subsolver initialization (#4731)

This decouples asserting a formula with initialization (previously it was a complex process to assert a formula due to having to clone/export to a new ExprManager). Now it is trivial.

This commit fixes an unintended consequence of the previous complications. Previously, SmtEngine::setOption would be set after asserting formulas to an SmtEngine subsolver, which is technically incorrect, as options should be finalized before the first assert.

This is required for further cleaning up of options listeners.