[imp] Alternate IMAP SQL Cache example please? :)

[Rick Romero]

Hi,

I have a 'remote' server that has a local readonly SQL repliaca, and I'd
like the IMAP Cache writes to be SQL locally.

backends.php refers to:
  - Horde_Imap_Client_Cache_Backend object: Directly configure the
 *           caching backend to use. For advanced users only.

Unfortunately I'm not advanced enough to know how to utilize that.  Can
someone provide an example I could modify? Memcache doesn't seem to be
doing a very good job of caching.

Thanks,
Rick