Fix repeating "Unknown blob for key" WARN logs after cached blob is deleted

If a Document's cached blob(s) are deleted, processes that check for existent blobs (e.g. orphan blob garbage collection) will log "Unknown blob for key: " on every execution (see LocalBlobStore.java:318). Those that recur often can result in this log being repeated hundreds or thousands of times which can bloat the server log.

Steps to Reproduce:

1. In an environment configured to use S3, create a Document either using or attaching a file (e.g. Picture) and confirm the blob is uploaded and stored.
2. Navigate to the instance's nuxeo.tmp.dir directory and locate the nxbincache folder where the cached binary is stored.
3. Delete the cached binary.
4. Run orphan blob GC process at least twice using the following command:

   curl -X DELETE -u Administrator:Administrator http://localhost:8080/nuxeo/api/v1/management/binaries/orphaned

Expected behavior: "Unknown blob for key: " WARN log is outputted on the first execution and is no longer generated following.

Actual behavior: "Unknown blob for key: " WARN log is outputted on each execution of the process.