Add DispatchStrategy to XioMessenger.

Joint implementation (by Matt and Marcus) of a queue-based
dispatch method for XioMessenger, based on Marcus' conclusion
that this will a) reduce dispatch latency (at the cost of
a context switch), but more importantly b) mask long latency
of some OSD operations currently inducing stalls.

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