Sync on `eth_newPayload`
When `eth_newPayload` arrives with a block whose parent is unknown, that block becomes a syncing candidate similar to `fcu` - this is useful for syncing side chains in an unstable network, where the non-canonical chains might become canonical after a while.

When syncing a side chain, the client should continue to respond as normal to canonical chain requests, ie it's the status of the canonical chain that determines the overall "syncing" status. 

Requests that specifically target the side chain (by selecting it via hash) should of course see `syncing` and similar responses.

Syncing can happen serially with any other  syncing going on - ie if another chain is being synced as a result of a newpayload/fcu, we can wait until that's done, at at least for the first version.