Allow a direct write mode

Change-Id: I2ec94437f24e0c7e4da91db03015e5e0c6057bc2
Reviewed-on: http://review.couchbase.org/25736
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>