ZEO Client Cache

  The client cache provides a disk based cache for each ZEO client.  The
  client cache allows reads to be done from local disk rather than by remote
  access to the storage server.

  The cache may be persistent or transient.  If the cache is persistent, then
  the cache file is retained for use after process restarts.  A non-
  persistent cache uses a temporary file.

  The client cache is managed in a single file, of the specified size.

  The life of the cache is as follows:

 -  The cache file is opened (if it already exists), or created and set to
    the specified size.

  - Cache records are written to the cache file, as transactions commit
    locally, and as data are loaded from the server.

  - Writes are to "the current file position".  This is a pointer that
    travels around the file, circularly.  After a record is written, the
    pointer advances to just beyond it.  Objects starting at the current
    file position are evicted, as needed, to make room for the next record
    written.

  A distinct index file is not created, although indexing structures are
  maintained in memory while a ClientStorage is running.  When a persistent
  client cache file is reopened, these indexing structures are recreated
  by analyzing the file contents.

  Persistent cache files are created in the directory named in the ``var``
  argument to the ClientStorage, or if ``var`` is None, in the current
  working directory.  Persistent cache files have names of the form::

    client-storage.zec

  where:

    client -- the client name, as given by the ClientStorage's ``client``
              argument

    storage -- the storage name, as given by the ClientStorage's ``storage``
               argument; this is typically a string denoting a small integer,
               "1" by default

  For example, the cache file for client '8881' and storage 'spam' is named
  "8881-spam.zec".