Swap out libatomicops for GCC builtin atomics.

In ceph::atomic_t.  CAS is overkill for a counter.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Initialize m in QueueStrategy.

Not a bug, but GCC is concerned.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Assigment optimizations XioMsg.

Remove memset (perfd slow).  Reorder assigments.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>