This makes a number of changes:
* Remove the dependency on marshal by using string.dump and loadstring.
* Use lua_tolstring rather than having Lua functions pass string lengths to C++.
* Move lua_api/l_async_events.* to cpp_api/s_async.*, where it belongs.
* Make AsyncWorkerThread a child of ScriptApiBase, this removes some duplicate functionality.
* Don't wait for async threads to shut down. (Is this safe? Might result in corruption if the thread is writing to a file.)
* Pop more unused items from the stack
* Code style fixes
* Other misc changes
Add curl_parallel_limit setting that will replace media_fetch_threads in
a later commit.
Fix a typo in MutexedQueue::pop_back() that made it impossible to compile
code that used this function. (Noticed this while implementing httpfetch.)
