[Feature] Flatten directory structure of backup store of ETCD BR

[abdasgupta]

Currently, directory structure of ETCD backup is like the following:
Current Directory Structure

v1
  - Backup-192345
      - Full-Snapshot-192345
      - Incremental-Snapshot-192355
      - Incremental-Snapshot-192365
      - Incremental-Snapshot-192375
  - Backup-192789
      - Full-Snapshot-192789
      - Incremental-Snapshot-192799
      - Incremental-Snapshot-192809
      - Incremental-Snapshot-192819

This feature will flatten the directory structure and coalesce all the snapshots under same directory:

v1
  - Full-Snapshot-revision-1
  - Incremental-Snapshot-revision-6-1 (Format- Incremental-Snapshot-revision-<Revision Number After Last Operation>-<Revision Number Before First Operation>)
  - Incremental-Snapshot-revision-12-6
  - Incremental-Snapshot-revision-14-12
  - Incremental-Snapshot-revision-18-14
  - Full-Snapshot-revision-18
  - Incremental-Snapshot-revision-22-18
  - Incremental-Snapshot-revision-27-22
  - Incremental-Snapshot-revision-32-27
  - Incremental-Snapshot-revision-38-32
  - Full-Snapshot-revision-38
  - Incremental-Snapshot-revision-41-38
  - Incremental-Snapshot-revision-43-41
  - Incremental-Snapshot-revision-48-43
  - Incremental-Snapshot-revision-52-48

This reorganization will make backup directory look simpler and easier to work with compaction.

Refer: gardener/etcd-druid#88

[abdasgupta]

Addressed here #301 . Closing it as the PR is merged