disconnect: don't touch easy-related state on disconnects

This was done to make sure NTLM state that is bound to a connection
doesn't survive and gets used for the subsequent request - but
disconnects can also be done to for example make room in the connection
cache and thus that connection is not strictly related to the easy
handle's current operation.

The http authentication state is still kept in the easy handle since all
http auth _except_ NTLM is connection independent and thus survive over
multiple connections.

Bug: http://curl.haxx.se/mail/lib-2014-08/0148.html
Reported-by: Paras S