We don't currently support deleting a DIB image while the builder
that built it is offline. The reason for this is to ensure that
we actually remove the files from disk on the builder. The mechanism
is for all other builders to defer handling "DELETING" image nodes
in ZK to the builder which built them.
This can be problematic if the builder is offline for an extended
period, or permanently.
To address this case without compromising the original goal, we now
let any builder delete the uploads and ZK nodes for a DIB build.
Subsequently, every builder will now look for DIB manifest directories
within its image-dir, and if it finds one that does not have a
corresponding ZK node, it garbage collects that image from disk.
Change-Id: I65efb31ca02cea4bcf7ef8f962a00b5263ccf69c
6386170914