Copy all checkpoints and writes from one thread to another.
Implementations must copy the complete parent chain (all
ancestor checkpoints and their checkpoint_writes) — copying
only the head checkpoint will leave the target thread with
DeltaChannel state that cannot be reconstructed (no path back
to a _DeltaSnapshot ancestor). Equivalently, the copy must
include enough ancestors that every DeltaChannel-backed key
has either a _DeltaSnapshot in channel_values somewhere in
the chain, or a complete write history back to the chain root.
