design proposal: lua-agent-check

[Robin H. Johnson]

As a followup to the http-agent-check design idea, I wondered if
implementing a general-case lua-agent-check mode would be beneficial.

lua-agent-check keyword would take one parameter, the name of a function
that can be called to determine the health of a server.

The finer details about the design model I'm not sure about yet.

Option 1: function is called for every (backend, server) tuple that
specifies the lua-agent-check keyword.
The Lua function would get two parameters:
- instance of Backend class that is being checked
- instance of Server class that is being checked

Option 2: function is called once for every backend. This would be
useful if you have an external system that knows the health for multiple
servers at once.