libceph: set CLOSED state bit in con_init

Once a connection is fully initialized, it is really in a CLOSED
state, so make that explicit by setting the bit in its state field.

It is possible for a connection in NEGOTIATING state to get a
failure, leading to ceph_fault() and ultimately ceph_con_close().
Clear that bits if it is set in that case, to reflect that the
connection truly is closed and is no longer participating in a
connect sequence.

Issue a warning if ceph_con_open() is called on a connection that
is not in CLOSED state.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>