Fix memory series indexing bug.

This fixes https://github.com/prometheus/prometheus/issues/381.

For any stale series we dropped from memory, this bug caused us to also drop
any other series from the labelpair->fingerprints memory index if they had any
label/value-pairs in common with the intentionally dropped series.

To fix this issue more easily, I converted the labelpair->fingerprints index
map values to a utility.Set of clientmodel.Fingerprints. This makes handling
this index much easier in general.

Change-Id: If5e81e202e8c542261bbd9797aa1257376c5c074
Merge "Fix memory series indexing bug."