Create a cache table per entity

I think we should make a cache_field_$entity table for each fieldable entity - i.e. cache_field_node, cache_field_user etc. This means table size (and hence overhead, key sizes etc.) will be a lot smaller, you'd be able to set up seperate cache bins in memcache per entity - good for when you have millions of records , and also backends like memcache which don't handle wildcard cache clearing very well. It also means we can use object id as the cache key, which should also make lookups a tiny bit faster than using composite strings like field:$obj_type:$id

Couple of very small kittens - hook_modules_installed() isn't called in install.inc, and hook_modules_uninstalled() is currently called after a module and all it's tables have been removed - not very useful if you need to check things like hook_fieldable_info() for the module being uninstalled.

Attached patch does a lot of the work - but things like simpletest are broken due to weird issues with which modules are actually loaded / installed during install. So marking postponed - I'm sure there's an issue somewhere but can't see it right now.