Rate-limit number of IOVs passed in any E-io operation per IOV_MAX

This avoids some weird errors on platforms which have a really tiny
limit for this.

Change-Id: I651d128314ffc2392c897edeec238adc499304b5
Reviewed-on: http://review.couchbase.org/43788
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>