storageImageSource: HasThreadSafeGetBlob -> true
storageImageSource: HasThreadSafeGetBlob -> true

Use a mutex to protect shared state in GetBlob() to enable concurrent
exectution of those.

Signed-off-by: Valentin Rothberg <c8d7c67387706f1534261b1aba83f5c355c00ff5@redhat.com>

containers/image,containers/image