Revise connection teardown.

Per Eyal, xio_connection_destroy may only be called in or after
XIO_SESSION_CONNECTION_TEARDOWN.  We were not handling the event
at all, but are after this change.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Merge branch 'xio-firefly-cmake-multi-ref' of https://github.com/linuxbox2/linuxbox-ceph into xio-firefly-cmake-multi-ref