In #4465, a regression was introduced by deleting the periodic call to
`engine_exchangeTransitionConfiguration` in the Nimbus light client.
This led to the light client never treating the EL as online and,
subsequently, not sending `engine_newPayload` requests for new blocks.
Strangely, `engine_forkchoiceUpdated` requests still make it through :)
Geth still requires both `engine_newPayload` and `fcU` to be called.
By restoring the `exchangeTransitionConfiguration` loop, `newPayload`
requests are once more issued to the EL.
9a4a5d0f88