ddfs_gc_main: use ets:match to find all of the pending blobs.
ddfs.py: when pushing to ddfs, only seek to the start if the source is seekable

Otherwise, print about the source not being seekable and then retry.
ddfs_gc_main: fix a bad pattern matching attempt.

We cannot do a lists:flatten and expect the items to match with [ObjName], in
this case, the flattening was unnecessary.
Merge pull request #563 from pooya/develop

ddfs_gc_main: use ets:match to find all of the pending blobs.