Protect map with a mutex

[dsymonds]

Go maps are not safe to mutate concurrent with other mutations or reads.

[grahamking]

Thanks! None of the version are thread safe, and I agree they should be. My tests never used more than one thread, so performance results shouldn't change. I'll merge this when I do the other two versions.

[jbarham]

Hah, you beat me to it. Here's another approach that wraps the cache map in a thread-safe container: https://github.com/jbarham/Key-Value-Polyglot/blob/master/memg.go