better handle context cancellations in the PBDagReader
better handle context cancellations in the PBDagReader

Good: If a previous read is canceled, we cancel the preloads that the read
triggered.
Bad: Future reads at that point will fail.

This fixes that issue.

License: MIT
Signed-off-by: Steven Allen <4068f0880b399410602d694b3cc711c8a8f4727e@stebalien.com>

ianopolous/go-ipfs,chriscool/go-ipfs,chriscool/go-ipfs,Luzifer/go-ipfs,ianopolous/go-ipfs,Luzifer/go-ipfs,chriscool/go-ipfs,ianopolous/go-ipfs,Luzifer/go-ipfs