Race condition in ubelt.Cacher

[Erotemic]

Describe the bug
There is a race condition in ubelt.Cacher if two processes or threads are trying to compute the same cached data.

This may or may not be ubelt's responsibility to deal with. We could offer mitigating features like optional use of safer when writing the cache file, or optional use of thread / inter process locks. This might add too much complexity to the cacher in terms of dependencies, but maybe not. Need to evaluate more.