Threading the lua execution

The LUA state is currently shared across all the connections in the MySQL Proxy to maintain a global state like

* internal counters
* the script cache
* proxy.global.*

For consistency the lua_state is protected by a mutex.

The shared lua_state should be turned into one independent lua_state per connection.

* use lua-lanes for maintaining the global scope
* reset lua_states if the connection doesn't need it anymore (cache it for reuse) to not loose the benifits of the script-cache