DfsStreamKey: Replace ForReverseIndex to separate metrics.

DfsStreamKey: Replace ForReverseIndex to separate metrics.

Keys used for identifying reverse indexes in the DfsBlockCache use a
custom subclass ForReverseIndex because there was no PackExt for them.
This conflates BlockCacheMetrics for reverse indexes with those for
packs, since the key falls back onto 0 when there is no extension.

Replace the custom ForReverseIndex with a DfsStreamKey usage to bring
keys for the new REVERSE_INDEX extension in line with INDEX and BITMAP
and separate reverse index and pack BlockCacheMetrics.

Change-Id: I305e2c16d2a8cb2a824855ea92e0c9a9b188fce5
Signed-off-by: Anna Papitto <annapapitto@google.com>