ddfs_gc: rebalance during the gc only if the DDFS_SPACE_AWARE is set.

By default, disco should not try to rebalance the blobs during the gc.  This is
consistent with the usage of this option for deciding whether we should
rebalance when we are putting a blob.
ddfs gc: fix the warning message for blob replication failure.
ddfs gc: add a more specific error message for ddfs put blob failure.
disco setting: unset ddfs_space_aware by default.

Space awareness can be enabled in the setting file, but by default, we want to
maintain the current behaviour and avoid unneeded re-replications.
Merge pull request #534 from pooya/ddfs_gc

Ddfs gc