Keymgr directory should default based on the state directory

Right now, if you override state_dir, the keymgr directory doesn't notice.

        // TODO #1185: perhaps the default `keystore_dir` should more generally be derived from
        // `state_dir`.
        //
        // Note this will involve writing a custom deserializer for StorageConfig (if
        // `storage.keystore.path` is missing from the config, it will need to be initialized using
        // the value we deserialized for `storage.state_dir` rather than a static default value)

See #1162 for another way of doing this.

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We plan, with #1202 (closed), to just remove the keystore directory configuration until we solve this issue. Once this is solved, we can revert #1202 (closed).