rsocket: Improve disconnect time under normal conditions

When both sides of a connection attempt to close at the same
time, one of the two sides can easily get an error when sending
a disconnect message.  This results in that side hanging
during close until the send times out.  (The time out is caused
by the remote side destroying its QP.)

We can reduce the chance of this occurring by immediately
assuming that the disconnect has been successful once we've
received the remote side's disconnect message.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>