Fix data races added in 161fc061d3106ef8fab81bbb823f8d2692d5b712

Change-Id: I7167472de3778fc76cc54dff6c864ed43b2052e9
diskpacked: re-use fds when possible when reading blobs from pack files.

We could actually do better than this and use the fd already open
from the still-appending file, or just keep one fd open per pack
file.

But this is a start.

Change-Id: I164a9bdf44987c3f057872cc7fdee1735f9e89a9