Use sync.Mutex rather than sync.RWMutex for simple projections

https://github.com/golang/go/wiki/CodeReviewConcurrency#rwmutex

Locking with RWMutex incurs extra overhead compared to plain sync.Mutex, and, furthermore, there might be some scalability issue with the current implementation of RWMutex in Go. Unless the case is very clear (such as an RWMutex used to synchronize many read-only operations each lasting hundreds of milliseconds or more, and the writes which require an exclusive lock seldom happen), there should be some benchmarks proving that RWMutex indeed helps to improve the performance.