Removing a docker component using its sha256 hash can cause performance degradation

There are several cases where removing a docker component (e.g. docker rmi) could block threads and subsequently impact the performance of the nexus server.

The cases are:
1. Trying to remove a docker component using a sha256 hash that no longer exists.
2. Removing a docker component that was created using a pre-release version of nexus using it's sha256 hash.

Symptoms are blocked threads and memory spike if nexus receives a number of docker digest based deletes.

Thread dump shows multiple threads with names similar to this one:

<command>sql.select from asset where bucket = :bucket and attributes[docker][content_digest] = :propValue</command> id=111 state=RUNNABLE