De-dupe selectable aggregate list in statistics API

Fixes bug 1298514

Discard full duplicates from the selectable aggregate list passed
to the statistics API.

Partial duplicates (i.e. matching func but different param) are
now allowed and supported by the storage drivers.

Change-Id: I5e2780befa9e265d5f1deb71b2aa375d05ca2e64
Merge "De-dupe selectable aggregate list in statistics API"