This patch allows for multiple RRD writer threads to service the queue.

This patch allows for multiple RRD writer threads to service the queue.

Now, rrdcached can have more simultaneous I/O requests into the kernel.
This may allow the kernel to re-order disk writes, resulting in better
disk throughput.  In practice, throughput is increased by 2-3x.

The flush (-f) timer maintenance has been moved from queue_thread_main
into its own thread.

Be more careful about when to use pthread_cond_signal vs _broadcast, since
multiple threads may be waiting on &queue_cond.

--kevin

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1665 a5681a0c-68f1-0310-ab6d-d61299d08faa