MB-11104 Don't allow checkpoint merge operation on active vbucket

The checkpoint merge operation on active vbucket might miss
snapshot_marker messages because we merge checkpoints by byseq
values.

Change-Id: Ibe2d526d38caf03d740d653fe09cb05332305bfd
Reviewed-on: http://review.couchbase.org/37354
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>