Tir is a simple Lua+Mongrel2 web framework that implements most of the features you need, and avoids the ones you're going to replace anyway, like ORM. It supports extensible templates; coroutine, handler, or callback state management, and is only 1300 lines of clean simple Lua code.
Tir applications take the model of a process per page making it easy to carve your application up across multiple servers and scale only the pieces that need it. If one part is sucking, just replace that part.
You can take a look at the features, download the latest and get started right now.