update "current_through_seq" on possible upr rollback
update "current_through_seq" on possible upr rollback

XDCR-over-upr receives failover_id message that contains real
start-seq that's used for upr stream that's consumed. Due to upr
rollback it can be much smaller than we expect. So it's important to
update our idea of how far we've replicated. Particularly so that xdcr
checkpointing doesn't create invalid checkpoints.

Change-Id: I9b7a1f3faab8d48929f0f64115d5abac81522f08
Reviewed-on: http://review.couchbase.org/35023
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,ceejatec/ns_server,mschoch/ns_server,membase/ns_server,membase/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,daverigby/ns_server,nimishzynga/ns_server,membase/ns_server,daverigby/ns_server,ceejatec/ns_server,mschoch/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server