XDCR - backfill req handler may not get correct throughSeqnos if pipeline is paused

For 7.0, we have the following lines:

https://github.com/couchbase/goxdcr/blob/d90bca90e8195c66250c434418825c744216420a/backfill_manager/backfill_request_handler.go#L440-L445

Normally, when raising a backfill, XDCR will go figure out the throughSeqno and compile a backfill task for each VB going from 0 to whatever the throughSeqno is.

When the pipeline is stopped and not running, it will retrieve 0. Instead, it should follow the other path of 

https://github.com/couchbase/goxdcr/blob/d90bca90e8195c66250c434418825c744216420a/backfill_manager/backfill_request_handler.go#L867

and capture checkpoint number, in case the pipeline is stopped.

For 7.0, it's not a huge issue because the first call only happens during pipeline runtime. (Can consider this for backport just in case).

However, this is a hole that should be fixed in preparation for 7.1 and the MB-9982 work.