indexer: Use atomic counters for scanner stats
indexer: Use atomic counters for scanner stats

Scan stats counters are updated by multiple scan workers
concurrently and hence synchronisation is required.

Change-Id: I92408530ddc7b98bcd4335b19e2cf22adc9f1891

jchris/indexing,jchris/indexing,jchris/indexing,jchris/indexing