Before this change when doing a sync with `--no-traverse` and
`--files-from` we could call `NewObject` a total of `--checkers` *
`--checkers` times simultaneously.
With `--checkers 128` this can exceed the 10,000 thread limit and
fails when run on a local to local transfer because `NewObject` calls
`lstat` which is a syscall which needs an OS thread of its own.
This patch uses a weighted semaphore to limit the number of
simultaneous calls to `NewObject` to `--checkers` instead which won't
blow the 10,000 thread limit and is far more sensible use of OS
resources.
Fixes#9073
Nick Craig-Wood
07e76419c9