Add iovused_high_watermark stat

New statistic which tracks the highest value of iovused - number of
iovecs used in a sendmsg().

Change-Id: I1d1a24477d94d39785ad11ae3f4c8ae8ff30a8d6
Reviewed-on: http://review.couchbase.org/38670
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>