[SPARK-17235][SQL] Support purging of old logs in MetadataLog

## What changes were proposed in this pull request?
This patch adds a purge interface to MetadataLog, and an implementation in HDFSMetadataLog. The purge function is currently unused, but I will use it to purge old execution and file source logs in follow-up patches. These changes are required in a production structured streaming job that runs for a long period of time.

## How was this patch tested?
Added a unit test case in HDFSMetadataLogSuite.

Author: petermaxlee <petermaxlee@gmail.com>

Closes #14802 from petermaxlee/SPARK-17235.