I am proposing to introduce a locking mechanism with RWMutex which helps to optimize read path by allowing multiple reads at once. Perform map initialization using sync.Once.

sync.Map does not have an api for measuring the length of it. Greatly complicates things for eviction implementation. Of course we could use sync/atomic and an int as an external counter but in this case I don't believe this additional complexity is worth it. Theoretically, sync.Map might be a better option

The Map type is optimized for two common use cases: (1) when the entry for a given key is only ever written once but read many times, as in caches that only grow, or (2) when multiple goroutines read, write, and overwrite entries for disjoint sets of keys. In these two cases, use of a Map may significantly reduce lock contention compared to a Go map paired with a separate Mutex or RWMutex.

My assumptions might be incorrect, feedback is welcome. Would gladly refactor this to use sync.Map if you see the benefit.

solves #128