MB-51298: Make checkpoint snapStartSeqno const

Make the checkpoint snapStartSeqno const as it should be immutable.

Change-Id: I75f37146e4a855f392bf73eb915c522727fd1715
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/173213
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>

engines/ep/src/checkpoint.h

@@ -408,10 +408,6 @@ class Checkpoint {
         return snapStartSeqno;
     }
 
-    void setSnapshotStartSeqno(uint64_t seqno) {
-        snapStartSeqno = seqno;
-    }
-
     uint64_t getSnapshotEndSeqno() const {
         return snapEndSeqno;
     }
@@ -641,7 +637,7 @@ class Checkpoint {
 
     EPStats& stats;
     const uint64_t checkpointId;
-    uint64_t                       snapStartSeqno;
+    const uint64_t snapStartSeqno;
     uint64_t                       snapEndSeqno;
 
     /// The maximum visible snapshot end (hides prepare/abort), this could be