CCBC-163: Fix high CPU usage

This fixes high CPU usage introduced in
8cd34a405523fc144937d13bed68a59f1a75f813.

The cause of the spike was only adding extra events if there was more
data to read/write. The fix is to always reset the requested events and
update the events as needed.

Change-Id: I2b59cdae04c0e07cbbca100462fd513f32e1f35f
Reviewed-on: http://review.couchbase.org/23692
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>