ddfs gc cleanup: modify error message for gc's normal termination.
ddfs: add histograms for traversing the tags or blobs.
ddfs_util: ignore unreadable disks for gc.

If a volume cannot be read, we still can proceed with the gc but we should
assume all nothing is in that volume.
disco_config: simplify the get_full_config function.
disco_config: introduce add_setting_if_missing function.

This function adds a setting if it is not already in the settings file.
Merge pull request #525 from pooya/ddfs_fixes

Ddfs fixes