Add lua bindings for space triggers, fixes gh-40.
on_replace triggers on struct lua_trigger.
Session and space triggers use the same technology.
Code cleanup.
space-luatriggers, gh-40, code review

Use common context for lua reference and for trigger state.
Wrap all calls to lua_call into a try/catch block.
Introduce LuarefGuard
Remove box_luactx, it's not adding any value now.
Implement multiple triggers. Reuse code in lua/trigger.cc.
Misc cleanups.
Rename a few files.