Acquire a reference count for manifests being read.

[nfachan]

The way it works now is:

• The ArtifactGatherer will start a manifest read on behalf of any job requests it gets, but only after the manifest has been read into cache.
• If there are multiple clients that all submit jobs using the same manifest simultaneously, they will each attempt to download the manifest independently. However, once the first one succeeds and asks to read the manifest, the manifest read will be shared by any other requests that come in.
• There is currently no way to cancel a manifest read.
• So, it's possible that a client may disconnect while a manifest read is outstanding. In this case, there will be no reference count held in the cache for the manifest, and in theory, the cache may remove the manifest while it's being read.