Whiteboard cacheing

[Alexander Larsson <alexl redhat com>]

The whiteboard cache changes didn't build for me, so i commited a fix
that I think will work. Its untested though, because i can't seem to
deploy without getting an OOM error atm.

Also, thinking about the caching during the night, wouldn't it make more
sense to actually cache the png images on disk? That way we'd use less
memory, let the kernel handle when and what to cache, and its still
faster to read the thumbnail file from disk than regenerating the full
image and then scaling it down. Especially since every client will be
reading every thumbnail.

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 Alexander Larsson                                            Red Hat, Inc 
                   alexl redhat com    alla lysator liu se 
He's a notorious zombie stage actor with a mysterious suitcase handcuffed to 
his arm. She's a mistrustful streetsmart Hell's Angel in the wrong place at 
the wrong time. They fight crime!