MB-100: started tracking timer lag histo

Which should be particularly helpful for detecting unhealthy erlang or
much overprovisioned VMs.

Change-Id: I85f22f9616da5ba738a99c9668dad90df52408c4
Reviewed-on: http://review.couchbase.org/30146
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>