[httpd] Multithread solution using worker threads instead of httpd threads

Using worker threads instead of httpd threads means that we, not libevent,
decide which requests get handled by which threads. This means that we can
make sure blocking requests (e.g. volume changes) don't get in the way of
realtime(ish) stuff like mp3 streaming.

Includes refactor of httpd_stream_file() since it was a bit of a monster.

src/worker.h

@@ -2,8 +2,10 @@
 #ifndef __WORKER_H__
 #define __WORKER_H__
 
+#include <event2/event.h>
+
 /* The worker thread is made for running asyncronous tasks from a real time
- * thread, mainly the player thread.
+ * thread.
 
  * The worker_execute() function will trigger a callback from the worker thread.
  * Before returning the function will copy the argument given, so the caller
@@ -19,6 +21,11 @@
 void
 worker_execute(void (*cb)(void *), void *cb_arg, size_t arg_size, int delay);
 
+/* Can be called within a callback to get the worker thread's event base
+ */
+struct event_base *
+worker_evbase_get(void);
+
 int
 worker_init(void);