SnapshotMarker from new checkpoint misses to set MARKER_FLAG_CHK

See https://review.couchbase.org/c/kv_engine/+/186221/3/engines/ep/tests/module_tests/dcp_stream_test.cc#5524 for details.
Also this might cause failures like seen in MB-55507.

As per comment in MB-55507:

Fix for MB-55465 up for review.
I'm keeping the fixes for MB-55465 and this one (MB-55507) separated because it's not 100% clear yet if the missing CHK flag in the marker that is causing the issue here is from the code path fixed in MB-55465.

Further investigation reveals that the crash observed here shouldn't be caused by MB-55465. Reason is that MB-55465 fixes a code path that cannot be executed for DCP Replication connections. While here we are failing at DCP Consumer in KV.

CC Daniel Owen