MB-12335: added support for not reloading buckets on cfg change

Now if config variable dont_reload_bucket_on_cfg_change is set to
true, bucket instances on entire cluster will not be restarted on
bucket config change like bucket priority change or eviction policy
change.

Setting it can be done by posting:

ns_config:set(dont_reload_bucket_on_cfg_change, true).

to /diag/eval. End this mode can be disabled by setting variable to
false. Once disabled all bucket instances that need to be restarted
will restart.

Change-Id: I951eeb4b69971a606eaef06af7a7252d380f25ff
Reviewed-on: http://review.couchbase.org/42311
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>