MB-12892: fixed cleanup race in bucket_info_cache

I believe issue in old code only affected case where notification
process had initial empty list of buckets. In such case first bucket
change event didn't cause reset of cached bucket info.

Instead of convincing ourselves that we only need to handle initial
situation, we came up with more arbuagly simpler and more robust
approach which serializes code which makes decision which buckets to
reset with caching code. And we keep track of precise bucket config
that was used to cache bucket info.

Change-Id: Ibd496002ee93391a37fa28816fb96cd3046cabce
Reviewed-on: http://review.couchbase.org/44195
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>