Handle highly concurrent conversion tasks

[imeoer]

These errors are likely to be thrown when acceld is handling highly concurrent conversion tasks:

convert in worker: convert image: failed commit on ref "converter-write-json-sha256:xxx": commit failed: rename /var/root/io.containerd.content.v1.content/ingest/yyy/data /var/root/io.containerd.content.v1.content/blobs/sha256/xxx: no such file or directory: unknown

convert in worker: push image: content digest sha256:zzz: not found

And containerd get locks slows down each task, containerd logs:

ERRO[2022-07-11T10:28:16.564583960Z] (*service).Write failed                       error="rpc error: code = Unavailable desc = ref default/1/convert-estargz-from-sha256:xxx locked for 5.41890936s (since 2022-07-11 10:28:10.633991309 +0000 UTC m=+98.894109053): unavailable" ref="convert-estargz-from-sha256:xxx"

Reproduciable steps:

1. acceld --config ./misc/config.yaml.estargz.tmpl
2. create a batch of conversion tasks:

for i in (seq 1 50)                                                                                                                                                                                                             
   accelctl task create localhost/busybox:latest
end

[imeoer]

Try to fix in containerd/containerd#7155

[Desiki-high]

@imeoer Can we use the singleflight in LocalAdapter.Dispatch to resolve the same conversion tasks?