Add time travel mock command

I don't like the tests to "sleep". Its better to just fast forward
the time

Change-Id: Ic995b412ce4dfe2aa712cd200bb0ae62a18e74e5
Reviewed-on: http://review.couchbase.org/27856
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>