CA-162029: Close FDs when deregistering an RRDD plugin

[simonjbeaumont]

No description provided.

[simonjbeaumont]

If the file still exists, it would open and add it to the cache in the current implementation. However, in reality, the file won't exist any more because the plugin is deregistered. The call to read_file will fail with ENOENT and this is handled by the exception handling logic in read_stats and no fd will be created and therefore none leaked.

I'd put some more effort into making the semantics of this cache a bit cleaner but it has changed upstream to use a much cleaner interface: https://github.com/xapi-project/rrd-transport/blob/master/lib/rrd_reader.ml.