Remove bookmarks hash

As part of the plugin and variables rework we need to remove the bookmarks hash

When a plugin is registered, it puts a reference to the variables it's going to use (sql level variables. like "set foo=bar") into a thing called the bookmarks hash. There is a mutex that protects this structure, because mysql supports runtime loading and unloading of modules so every reference to any plugin variable locks on the bookmarks mutex. We need to remove this structure since it is not needed
[10:46am] mtaylor: when we talk about getting rid of it, we're talking about not having that structure at all (because we don't need it)