libceph: split protocol reset bits out of reset_connection()

Move protocol reset bits into ceph_con_reset_protocol(), leaving
just session reset bits.

Note that con->out_skip is now reset on faults.  This fixes a crash
in the case of a stateful session getting a fault while in the middle
of revoking a message.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>