_gallery_block calls GalleryEmbed::handleRequest causing performance and functionality problems

If you have a gallery block active, then it's going to call _gallery_block at least once. Right now in 4.7, this means that it will call GalleryEmbed::handleRequest -- which in turn will run through the entire process of building a page, when all it needs is the side bar block HTML. This also appears to interfere with the Gallery page caching code because it tries to write the same entry to the database twice and gets a database error (I witnessed this on a user's box -- haven't reproduced it myself, but I can help you reproduce it if you want).

It would be much more efficient to get this result once during the main handleRequest call and store it for subsequent requests so that it doesn't do two complete page renders. Let me know if you need more info.