[0.12] Duplicate CREATE DATABASE with change in SHARD DURATION fails silently

[rkuchan]

As expected, when I duplicate a database and retention policy I receive no error:

> CREATE DATABASE mydb WITH DURATION 1d REPLICATION 1 SHARD DURATION 30m NAME myrp
> CREATE DATABASE mydb WITH DURATION 1d REPLICATION 1 SHARD DURATION 30m NAME myrp
>

Also as expected, when I create the same database and retention policy but change REPLICATION or DURATION, I receive an error:

> CREATE DATABASE mydb WITH DURATION 1d REPLICATION 1 SHARD DURATION 30m NAME myrp
> CREATE DATABASE mydb WITH DURATION 1d REPLICATION 2 SHARD DURATION 30m NAME myrp
ERR: retention policy conflicts with an existing policy
> CREATE DATABASE mydb WITH DURATION 2d REPLICATION 1 SHARD DURATION 30m NAME myrp
ERR: retention policy conflicts with an existing policy

I do not receive an error if I create the same database and retention policy but change the SHARD DURATION, and the retention policy's SHARD DURATION remains the initial duration:

> CREATE DATABASE mydb WITH DURATION 1d REPLICATION 1 SHARD DURATION 30m NAME myrp
> CREATE DATABASE mydb WITH DURATION 1d REPLICATION 1 SHARD DURATION 40m NAME myrp
> show retention policies on mydb
name    duration    shardGroupDuration  replicaN    default
myrp    24h0m0s     30m0s               1           true