archive-get refactoring

[gsmolk]

Done:

1. Batching: --batch-size option support
2. Multi-threading: -j option support
3. Prefetching: automatically enabled if batching is used. New option: --prefetch-dir
4. .partial and .gz.partial files are handled
5. Validation of prefetched WAL files: if prefetching is used, then prefetched WAL segment is parsed before feeding it to recovery. Flag --no-validate-wal disables this.
6. Retrying: if requested file do not exists, invalid or we encountered some error while copying it, we retry copying several times (3) before giving up

TODO:

1. help
2. documentation
3. tests