Support Automatic Key Rotation via the Metastore

This depends on #23

We should allow automatic rotation to be configured in the `MetaStore` such that an attempt to use an overly old key automatically (and silently) triggers creation of a new key. This will maintain the invariant that "No data is encrypted with a key older than `$ROTATION_TIME`." while not causing needless rotations when no encryption is occurring.

首先阅读 issue #23 中的依赖项，以及 MetaStore 配置和密钥使用路径。跟踪加密尝试如何选择密钥，并确定密钥何时过旧。完成的标准是：轮换可配置，只有在加密使用过旧密钥时才静默执行，并且在不进行不必要轮换的情况下保持所述的密钥年龄不变量。

由索引模型根据 Issue 内容生成。