lua: Reworked Lua destination to be a threaded destination.

Lua was the simplest destination driver of all kind, but because it lacked any
kind of queue management or synchronization, it was easy to get it killed by
several parallel input connections. Not it was fixed by switching to threaded
destination driver.
Merge pull request #59 from talien/f/lua-threaded-dest-driver

lua: Reworked Lua destination to be a threaded destination.