Sync on eth_newPayload

[arnetheduck]

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.