[FTS] moss store data race between collection level Get() & background merger

Wrote a simple unit test to just load 1M items while simultaneously issuing collection level Get() operations and hit a data race between the background collection merger go routine and the front end Get()

WARNING: DATA RACE

Write at 0x00c420dc6c10 by goroutine 7:  github.com/couchbase/moss.(*collection).mergerNotifyPersister()      github.com/couchbase/moss/collection_merger.go:280 +0x676  github.com/couchbase/moss.(*collection).runMerger()      github.com/couchbase/moss/collection_merger.go:132 +0x454

Previous read at 0x00c420dc6c10 by goroutine 6:  github.com/couchbase/moss.(*segmentStack).get()      github.com/couchbase/moss/segment_stack.go:106 +0x81  github.com/couchbase/moss.(*segmentStack).Get()      github.com/couchbase/moss/segment_stack.go:71 +0xae  github.com/couchbase/moss.(*collection).get()      github.com/couchbase/moss/collection.go:640 +0x40b  github.com/couchbase/moss.(*collection).Get()      github.com/couchbase/moss/collection.go:259 +0xe5  github.com/couchbase/moss.Test_JustLoad1Mitems()      github.com/couchbase/moss/persister_test.go:1201 +0x51b  testing.tRunner()      }}{{