Prune checkpoints for the given threads.
"delete" removes every checkpoint, write and blob of the threads, in
one transaction.
"keep_latest" keeps the latest checkpoint of each namespace (the one
get_tuple loads, plus its parent when it predates v4 and has pending
sends), with its pending writes and the blobs it references, and
deletes the rest. A kept checkpoint whose parent it deletes is left with
no parent. A subgraph call's namespace is deleted with the root
checkpoint the call started from, except a checkpointer=True
subgraph's, which keeps its latest checkpoint. Each thread is pruned in
its own transaction, on one snapshot of it, so a run appending to the
thread meanwhile is safe; forking or replaying from a checkpoint being
deleted is not. A thread that fails because another transaction changed
it leaves the threads before it pruned, and retrying is safe.
"keep_latest" refuses threads that use a DeltaChannel, since it
rebuilds the latest value from older checkpoints: if any requested
thread does, nothing is pruned. A thread that starts using one before
its turn in the batch is refused then, after the threads before it.
The thread IDs to prune.
"keep_latest" or "delete".
