Adapt xdcr to UPR protocol changes.

Protocol changes in a nutshell:

 - snapshot marker is sent at the beginning of a snapshot (not at the
   end) and carries sequence numbers of the snapshot's start and end

 - stream request command takes snapshot start and snapshot end
   sequence numbers and doesn't require high sequence number

Change-Id: Idd97773f3c8594503f6e79447a7257c361c4515b
Reviewed-on: http://review.couchbase.org/36612
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>