implement transient file tracking and cleanup

[raulk]

We use a mount.Upgrader to mirror remote shards into local files. Those local files are currently (a) regular temporary files, and they are (b) not managed. We need to do the following in the upgrader:

• pass in the transient root directory to upgraders, so that transients are created inside that directory (referred to as "scratch space")
• replace the usage of temporary files with regular files under that directory.
• add a method DeleteTransient() error in the upgrader that deletes the transient file under the lock.

In the DAG store management layer, we need to do the following:

• expose a method GC() error that deletes the transient on all shards whose refcount is 0 (unused, i.e. not serving).
  • done in implement DAGStore#GC. #52.

[raulk]

The DAGStore#GC() error method was not implemented in #49. Reopening.