Support for offsets as startPosition

[mikespicer]

The startPosition parameter currently supports begin/end and time. For jobs that need to restart from a known position there is a requirement for offsets similar to time. While time is similar to offset there is a concern that it is not as accurate/explicit as offsets for high rate feeds that need to guarantee processing all data after a restart where millisecond precision is not sufficient. James had raised that offsets are partition based so we will need to take that into account.