Make sure that clients can't access buffers belonging to other users

Make sure that clients can't access buffers belonging to other users

A manipulated, but properly authenticated client was able to retrieve
the backlog of other users on the same core in some cases by providing
an appropriate BufferID to the storage engine. Note that proper
authentication was still required, so exploiting this requires
malicious users on your core. This commit fixes this issue by ensuring
that foreign BufferIDs are off-limits.