api: add memsize() methods to ParamValueList and ImageSpec

[lgritz]

To aid some memory accounting, add a method to ParamValueList and ImageSpec that return the memory cost (the size of the struct and all the allocated things hanging off of it). We're trying to be more careful about understanding how much memory is used by having many copies of ImageSpec's lying around.

Have ImageCache stats estimate how much mem is tied up in ImageSpecs.