CREATE RETENTION POLICY not properly checking SHARD DURATION

[e-dard]

When recreating the same retention policy the SHARD DURATION is not checked properly. If it's different to the existing SHARD DURATION an error should be returned, but one is not.

> CREATE RETENTION POLICY rp0 ON db DURATION 1h REPLICATION 1 SHARD DURATION 60m
> CREATE RETENTION POLICY rp0 ON db DURATION 1h REPLICATION 1 SHARD DURATION 61m
>
> SHOW RETENTION POLICIES ON db
name    duration    shardGroupDuration  replicaN    default
default 0       168h0m0s        1       true
rp0 1h0m0s      1h0m0s          1       false

>