Crash in sizeBelowThreshold

[wenyue]

I found a bug that can cause a crash. In the directory sizeBelowThreshold function, we call the EnumLinksAsync function, then call parallelShardWalk, and the sizeBelowThreshold function may be end if we have already fetched enough shards to assert we are above the threshold. At this time, parallelShardWalk may still running the nextShard.walkChildren.
Next, because sizeBelowThreshold has been completed, we can continue to modify direcotry. At this time, walkChildren will cause a crash because the hamt ds.childer has been modified.

An example is:
Call direcotry.unlink(), and then immediately call direcotry.addChild(). If run many times, it may trigger a crash.

[welcome]

Thank you for submitting your first issue to this repository! A maintainer will be here shortly to triage and review.
In the meantime, please double-check that you have provided all the necessary information to make this process easy! Any information that can help save additional round trips is useful! We currently aim to give initial feedback within two business days. If this does not happen, feel free to leave a comment.
Please keep an eye on how this issue will be labeled, as labels give an overview of priorities, assignments and additional actions requested by the maintainers:

• "Priority" labels will show how urgent this is for the team.
• "Status" labels will show if this is ready to be worked on, blocked, or in progress.
• "Need" labels will indicate if additional input or analysis is required.

Finally, remember to use https://discuss.ipfs.io if you just need general support.

[wenyue]

This is my fix, please take a look.
wenyue@af6f2b0

[lidel]

@wenyue are you willing to open a PR with the fix and a test?

Triage note: we will look into this in spare time.