Fetch provisioners list from stake contract only if the list is updated

[goshawk-3]

Describe the bug
GetProvisioners call consumes more than 4s (up to 15s on low-spec nodes) allegedly due to serialization in VM. Although this is not supposed to be the case with the optimized VM (next version), this should be handled gracefully from client-side (Golang part).

To Reproduce

1. Introduce 2000-size allow list
2. Call Accept/Finalize (as a result this calls GetProvisioners).

Expected behaviour
Until next VM is benchmark-ed and fully integrated, dusk node should not call grpc.GetProvisioners if provisioners list is not updated.