The concurrent walker created by walk() only stopped when the callback
returned an error or the whole tree had been listed. Cancelling the
context (for example via the rc job/stop endpoint for an async
operations/size or recursive operations/list call) was therefore
ignored: the checkers kept pulling list jobs from the channel and kept
listing the entire tree, burning CPU and making job cancellation
useless for every backend without a native ListR implementation.
Make every checker select on ctx.Done() so a cancelled walk shuts down
promptly through the existing quit/drain path and reports the context
error. Also check the context between directory read chunks in the
local backend so a single huge directory does not block cancellation.
