XioConnection request tracking 1.

Track requests using hook ctor/dtor.  Lockless, portal thread
provides atomicity.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Restore zero-copy buffers in OSD fast path.

This change restores volatile sharing semantics in the Message
decode path, and also in the OSD write path for FileStore/FileJournal.

This can be verified with a breakpoint set at the clone/COW case in
buffer::ptr::strong_claim (currently buffer.cc:690).

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