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>