MB-5307 Don't log useless message about changed config.

During rebalance we change 'buckets' key a lot which results in lots
of these messages in the log file. At the same time, they are
completely useless.

Change-Id: I478ec80f0e89042792ef554698107bacd6a5275f
Reviewed-on: http://review.couchbase.org/24258
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-5307 Log less when deciding if compaction is needed.

Basically all we need to log is data and disk sizes. Everything else
about resulting decision can be easily inferred from this data.

Change-Id: I60cf6cdc1f9b4b293abd8c9f7c1f95e22a7fd0a7
Reviewed-on: http://review.couchbase.org/24279
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-7671 Check fragmentation before compacting vbucket.

When we decide to compact or not to compact a bucket we just take a
random sample of vbuckets and check aggregated fragmentation of these
vbuckets. Although not very likely, it's possible that we'll decide to
compact entire bucket when only few vbuckets have high
fragmentation. To avoid an overhead of compacting vbuckets that don't
really need thie, now we check fragmentation of every single vbucket
before compacting it.

Change-Id: Iaa719c45800caf46c726cd06eb8afb2113b47887
[alk@tut.by: added jira ticket]
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/24280
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>