Add missing log for failed GC record unmarshalling in purgeStore()

[vipocenka]

This PR replaces a // TODO comment with an actual log statement in the purgeStore() method of the dsAddrBookGc:

Change summary:

• Added a log.Warnf when proto.Unmarshal fails during garbage collection:

  log.Warnf("failed to unmarshal record during GC purge: key=%s, err=%v", result.Key, err)

[MarcoPolo]

Thanks :) are you using the pstoreds package by any chance?

[vipocenka]

@MarcoPolo
I'm not using the pstoreds package myself — I just came across the TODO while reviewing the code and thought it would be helpful to replace it with a proper log statement.