hrtime: Don't return small values

Some functionality of the library assumes that the current timestamp is
well above 0. With various clock sources - especially those which depend
on manually starting/stopping a process-local timer. This ensures that
the timestamp is offset by a fixed number, so that confusion will not
happen.

Change-Id: I34f68dc56b3c55489f588bef2cde5e02ed7e5932
Reviewed-on: http://review.couchbase.org/44251
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>