MB-8665: don't send server version in headers

It has it's performance cost (about 60us on my box) for every single
request and doesn't really help anybody.

Change-Id: I07a50e4d5448778f26ce4817eff9ebe7d70457fc
Reviewed-on: http://review.couchbase.org/28953
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>