CBSE-1437/MB-12371: Only keep the highest seqno in the meta file

The bug is caused by that we kept the newer seqno instead of highest
seqno in the meta file. So the client will send smaller start seqno
to server in subsequent runs.

Change-Id: Ic986de8df58bd167849898c420f3b6863f373a06
Reviewed-on: http://review.couchbase.org/42225
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Steve Yen <steve.yen@gmail.com>