Figure out how to do caching

[mithrandi]

Something like txsni.maputils.Cache is too simplistic, because it assumes the underlying store will never change, which is obviously false in the case of txacme.

[mithrandi]

I think the caching logic is necessarily going to be backend-specific; and while we could implement an API for ICertificateStore to communicate with a cache, I think it would be better to have this encapsulated in the store instead. Thus we might have CachingDirectoryStore which, for example, watches the filesystem with inotify for changes and keeps the cache fresh that way.