DDFS GC should reuse the the blob_map and tag_map if gc fails in a later stage.

[pooya]

If ddfs gc fails for any reason in a stage after build_map, it should not give up the blob_map and tag_map ets tables. Chances are the admin is going to restart the gc very soon and the maps can be reused for the next gc.
These ets tables should be deleted only when the gc terminates successfully, or when they expire.

This means we also have to set an upper limit on the length of each stage of the gc.
There are some notes about handling of the ets tables in this blog post: http://steve.vinoski.net/blog/2011/03/23/dont-lose-your-ets-tables

It also makes sense to add an option to discard these ets tables and start the gc from scratch.