MB-8665: introduced menelaus_web_cache and used it for versions

/pools is potentially accessed frequently as part of clients
bootstrapping and I've found component versions that are rendered as
part of that response to take significant amount of CPU.

Given versions don't change at all we can cache this info
aggressively. For caching I'm using same approach as we've used in
vbucket_map_mirror.

Change-Id: If83991e172004cc8df915dd392d200c1988d7e92
Reviewed-on: http://review.couchbase.org/28954
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>