Support `set[Node]Timestamp!` (maybe)

[Affie]

For DFG v1.0.0, I'm removing setTimestamp[!] 🦨 .

It is best to create the node with the correct timestamp from the start or use merge to replace the whole node.

For v1.x we can look at expanding with set[Node]Timestamp!

• 👣 Accessors.jl

Originally posted by @Affie in #315