Prefer lcb_assert() to abort()

Change-Id: Ie824d6e75f24876e31a0eeede90a66d3a3380ccd
Reviewed-on: http://review.couchbase.org/28756
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
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