S3Guard: Implement time-based (TTL) expiry for Authoritative Directory Listing

Similar to HADOOP-13649, I think we should add a TTL (time to live) feature to the Dynamo metadata store (MS) for S3Guard.

This is a similar concept to an "online algorithm" version of the CLI prune() function, which is the "offline algorithm".

Why:
1. Self healing (soft state): since we do not implement transactions around modification of the two systems (s3 and metadata store), certain failures can lead to inconsistency between S3 and the metadata store (MS) state. Having a time to live (TTL) on each entry in S3Guard means that any inconsistencies will be time bound. Thus "wait and restart your job" becomes a valid, if ugly, way to get around any issues with FS client failure leaving things in a bad state.
2. We could make manual invocation of `hadoop s3guard prune ...` unnecessary, depending on the implementation.
3. Makes it possible to fix the problem that dynamo MS prune() doesn't prune directories due to the lack of true modification time.

How:
I think we need a new column in the dynamo table "entry last written time". This is updated each time the entry is written to dynamo.
After that we can either
1. Have the client simply ignore / elide any entries that are older than the configured TTL.
2. Have the client delete entries older than the TTL.

The issue with #2 is it will increase latency if done inline in the context of an FS operation. We could mitigate this some by using an async helper thread, or probabilistically doing it "some times" to amortize the expense of deleting stale entries (allowing some batching as well).

Caveats:

• Clock synchronization as usual is a concern. Many clusters already keep clocks close enough via NTP. We should at least document the requirement along with the configuration knob that enables the feature.