Add support for evict

Given that the mock don't have a concept of whats in
memory and whats not we'll always return success if
the key's in there.

Change-Id: I7797e641dc12186aef5d7ecec6a67fb2392cc231
Reviewed-on: http://review.couchbase.org/29148
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>