CCBC-263 Skip unfinished SASL commands on rebalance

During rebalance, it is possible that the newly added server doesn't
have chance to finish SASL auth before the cluster will push config
update, in this case packet relocator messing cookies.

Also the patch makes sure that SASL command/cookie isn't mixing with
other commands

Change-Id: Ic3fb01b4a39d2e8e68c690c264a2918d631d9602
Reviewed-on: http://review.couchbase.org/28744
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>