block cache multiple per region instead of per single block

Hi,

while rendering blocks, some useless block cache queries could be prevent by retrieving cache per block region rather than per block individually.

But of course, a better solution should be to retrieve all blocks caches at once...
And unfortunetly, as block rendering process is designed (per region individually rather than a list of regions), we can't load all blocks caches at once without rewriting it completly...

Thanks