fix(datastore): do not override DLB replica DSNs in for loop

What does this MR do?

Variables in for loops are reused for each iteration, and in this specific case, we were passing its address (&dsn) to the lb.connector.Open function. Since dsn is a single variable reused across iterations, the pointer &dsn will always point to the same memory location, which contains the last value assigned to dsn by the loop. Note that the actual connection logic is correct, as each connection is open using the correct DSN. The problem is in the logging. See below.

Before

INFO[0000] enabling database load balancing with service discovery  go_version=go1.21.5 instance_id=c1a11593-c88c-468c-9f83-9a7b9c24d52c nameserver=localhost port=8600 record=replica.pgbouncer.service.consul replica_check_interval_s=15 version=v4.8.0-gitlab-2-g85a3e9896
INFO[0000] resolving replicas with service discovery     component=registry.datastore.DBLoadBalancer go_version=go1.21.5 instance_id=c1a11593-c88c-468c-9f83-9a7b9c24d52c version=v4.8.0-gitlab-2-g85a3e9896
INFO[0000] opening replica connection                    address="127.0.0.1:6432" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=0 instance_id=c1a11593-c88c-468c-9f83-9a7b9c24d52c total=4 version=v4.8.0-gitlab-2-g85a3e9896
INFO[0000] opening replica connection                    address="127.0.0.1:6435" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=1 instance_id=c1a11593-c88c-468c-9f83-9a7b9c24d52c total=4 version=v4.8.0-gitlab-2-g85a3e9896
INFO[0000] opening replica connection                    address="127.0.0.1:6433" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=2 instance_id=c1a11593-c88c-468c-9f83-9a7b9c24d52c total=4 version=v4.8.0-gitlab-2-g85a3e9896
INFO[0000] opening replica connection                    address="127.0.0.1:6434" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=3 instance_id=c1a11593-c88c-468c-9f83-9a7b9c24d52c total=4 version=v4.8.0-gitlab-2-g85a3e9896
INFO[0000] updating replicas list                        added="[127.0.0.1:6434 127.0.0.1:6434 127.0.0.1:6434 127.0.0.1:6434]" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 instance_id=c1a11593-c88c-468c-9f83-9a7b9c24d52c removed="[]" version=v4.8.0-gitlab-2-g85a3e9896

Note that we open connections against each host as expected, but then we log 4 times the last host as added.

After

INFO[0000] enabling database load balancing with service discovery  go_version=go1.21.5 instance_id=f318daaf-212c-4075-ac9b-e6d2e58705c1 nameserver=localhost port=8600 record=replica.pgbouncer.service.consul replica_check_interval_s=15 version=v4.8.0-gitlab-2-g85a3e9896.m
INFO[0000] resolving replicas with service discovery     component=registry.datastore.DBLoadBalancer go_version=go1.21.5 instance_id=f318daaf-212c-4075-ac9b-e6d2e58705c1 version=v4.8.0-gitlab-2-g85a3e9896.m
INFO[0000] opening replica connection                    address="127.0.0.1:6435" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=0 instance_id=f318daaf-212c-4075-ac9b-e6d2e58705c1 total=4 version=v4.8.0-gitlab-2-g85a3e9896.m
INFO[0000] opening replica connection                    address="127.0.0.1:6433" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=1 instance_id=f318daaf-212c-4075-ac9b-e6d2e58705c1 total=4 version=v4.8.0-gitlab-2-g85a3e9896.m
INFO[0000] opening replica connection                    address="127.0.0.1:6434" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=2 instance_id=f318daaf-212c-4075-ac9b-e6d2e58705c1 total=4 version=v4.8.0-gitlab-2-g85a3e9896.m
INFO[0000] opening replica connection                    address="127.0.0.1:6432" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 index=3 instance_id=f318daaf-212c-4075-ac9b-e6d2e58705c1 total=4 version=v4.8.0-gitlab-2-g85a3e9896.m
INFO[0000] updating replicas list                        added="[127.0.0.1:6435 127.0.0.1:6433 127.0.0.1:6434 127.0.0.1:6432]" component=registry.datastore.DBLoadBalancer go_version=go1.21.5 instance_id=f318daaf-212c-4075-ac9b-e6d2e58705c1 removed="[]" version=v4.8.0-gitlab-2-g85a3e9896.m

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