This removes the list of in-flight blobs from the master index and
instead keeps a list of "known" blobs in the Archiver. "known" here
means: either already processed, or included in an index. This property
is tested atomically, when the blob is not in the list of "known" blobs,
it is added to the list and the caller is responsible to make this
happen (i.e. save the blob).
This adds code to the master index to allow saving duplicate blobs
within the repacker. In this mode, only the list of currently in flight
blobs is consulted, and not the index. This correct because while
repacking, a unique list of blobs is saved again to the index.
Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).
This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.
