Concurrent update of aptly repo.

[goldendeng]

https://github.com/aptly-dev/aptly/blob/b3d9055059e24e1d96a462dc080f46071ced81b7/deb/local.go#L160

In the concurrent update scenario of aptly's repo, it seems to me that the existing software package index update process of aptly's repo may be missed. In fact, I have encountered it when using the etcd database. I wonder whether the existing software package index update process is deeply bound with the batch operation of leveldb?