Fixes #1518 Data races detected in channel_cache.go
Fixes #1518 Data races detected in channel_cache.go

https://github.com/couchbase/sync_gateway/issues/1518

tophatch/sync_gateway,tophatch/sync_gateway,ceejatec/sync_gateway,ceejatec/sync_gateway,tophatch/sync_gateway,couchbaselabs/sync_gateway,couchbaselabs/sync_gateway,tophatch/sync_gateway,couchbaselabs/sync_gateway,ceejatec/sync_gateway,ceejatec/sync_gateway,couchbaselabs/sync_gateway