cache sql query results using dogpile/beaker

[joetsoi]

I don't believe we actually cache any of our sql queries, as a result we end up with repeated requests, for example model.User.get() is run when we run check_access anywhere in the templates, so on some pages we end up fetching the user a couple of dozen times.

I haven't had an indepth look, but we have the beaker caching middleware enabled, but as far as I can tell, we don't actually use it anywhere to cache results returned by sqlalchemy. The sqlalchemy docs now recommend using dogpile over beaker I think both were at some point written by the sqlalchemy guys

http://beaker.readthedocs.org/en/latest/configuration.html
http://dogpilecache.readthedocs.org/en/latest/

[rossjones]

This is worse than it appears, as we were actively avoiding the local sqlalchemy 'cache'.

For .get() calls this should be fixed by #2824 and #2825 where the ID is passed (because the 'cache' is an identity map on the PK).