FileCache:  Removed the reuse= argument.  IIRC, it made some sense
when different cache schemes were first being tried, but complicated
the logic now to no good end.

Nuked the maddening log warnings about reuse=True.  They were never
helpful and were often confusing.

Added info-level log messages to record the path of the client cache
file, whether it's persistent or temporary, and if it's persistent
whether we're creating or reusing it.

Minor changes to comments.