buffer: don't use ringbuffer in chunk

We can't use a ringbuffer on the chunk because it implies the
consumer would write to it to update the read position, which we
can't do because the chunk is read-only and might even be shared.
Go back to offset/size pairs, which can sortof do the same thing
if we want later when we keep a non-shared read pointer in the
consumer.
Keep alsa timestamp around and filled state for future.
mmap the input port meta/data/chunk as read-only.
Only do clock update requests when asked.

spa/include/spa/buffer/meta.h

@@ -25,7 +25,6 @@ extern "C" {
 #endif
 
 #include <spa/utils/defs.h>
-#include <spa/utils/ringbuffer.h>
 #include <spa/support/type-map.h>
 
 /** \page page_meta Metadata