Add BucketLoader.last_summarize_rec

When working on the compactor, it was discovered that the Redis LINSERT
command acts on values, not indices, and that there is no Redis command
that inserts values at a specific index.  To accommodate this, it is
necessary to do a couple of things:

1. Ensure each 'summarize' record is unique, by adding a UUID.
2. Ensure that we have the raw value of the last summarize record, not
   just its index.