Threaded IO: implement handleClientsWithPendingWritesUsingThreads().

This is just an experiment for now, there are a couple of race
conditions, mostly harmless for the performance gain experiment that
this commit represents so far.

The general idea here is to take Redis single threaded and instead
fan-out on expansive kernel calls: write(2) in this case, but the same
concept could be easily implemented for read(2) and protcol parsing.

However just threading writes like in this commit, is enough to evaluate
if the approach is sounding.

src/server.c

@@ -1981,9 +1981,6 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
             flushAppendOnlyFile(0);
     }
 
-    /* Close clients that need to be closed asynchronous */
-    freeClientsInAsyncFreeQueue();
-
     /* Clear the paused clients flag if needed. */
     clientsArePaused(); /* Don't check return value, just use the side effect.*/
 
@@ -2075,7 +2072,12 @@ void beforeSleep(struct aeEventLoop *eventLoop) {
     flushAppendOnlyFile(0);
 
     /* Handle writes with pending output buffers. */
-    handleClientsWithPendingWrites();
+    /* XXX: Put a condition based on number of waiting clients: if we
+     * have less than a given number of clients, use non threaded code. */
+    handleClientsWithPendingWritesUsingThreads();
+
+    /* Close clients that need to be closed asynchronous */
+    freeClientsInAsyncFreeQueue();
 
     /* Before we are going to sleep, let the threads access the dataset by
      * releasing the GIL. Redis main thread will not touch anything at this
@@ -2861,6 +2863,7 @@ void initServer(void) {
     slowlogInit();
     latencyMonitorInit();
     bioInit();
+    initThreadedIO();
     server.initial_memory_usage = zmalloc_used_memory();
 }