* purge_old_snapshot_archives is changed to take an extra argument 'maximum_snapshots_to_retain' to control the max number of latest snapshot archives to retain. Note the oldest snapshot is always retained as before and is not subjected to this new options.
* The validator and ledger-tool executables are modified with a CLI argument --maximum-snapshots-to-retain. And the options are propagated down the call chains. Their corresponding shell scripts were changed accordingly.
* SnapshotConfig is modified to have an extra field for the maximum_snapshots_to_retain
* Unit tests are developed to cover purge_old_snapshot_archives