feat(pendingsnapshots): add pending snapshots on CVR by talking to peer replicas

[mittachaitu]

What this PR does / why we need it:
This PR adds the pending snapshots under CVR status.pendingsnapshots only if CVR underwent Rebuilding. This PR is continuation of PR #1639.

How Does It Gets Pending Snapshots:
The replica which is undergoing rebuilding will get the list of pending snapshots from other peer CVR's Snapshot information.

Sample YAML output when Rebuilding is in Progress:

apiVersion: openebs.io/v1alpha1
  kind: CStorVolumeReplica
  metadata:
    annotations:
      cstorpool.openebs.io/hostname: e2e1-node3
      isRestoreVol: "false"
      openebs.io/storage-class-ref: |
        name: single-sc
        resourceVersion: 41963
    creationTimestamp: "2020-04-01T09:47:53Z"
    finalizers:
    - cstorvolumereplica.openebs.io/finalizer
    generation: 9
    labels:
      cstorpool.openebs.io/name: cstor-sparse-pool-uw8i
      cstorpool.openebs.io/uid: ccc2be88-fa06-4cb6-9f64-ef5e6fd196c4
      cstorvolume.openebs.io/name: pvc-ffb6210a-dae5-4186-899f-f7f8333d24dd
      openebs.io/cas-template-name: cstor-volume-create-default-1.8.0
      openebs.io/persistent-volume: pvc-ffb6210a-dae5-4186-899f-f7f8333d24dd
      openebs.io/version: 1.8.0
    name: pvc-ffb6210a-dae5-4186-899f-f7f8333d24dd-cstor-sparse-pool-uw8i
    namespace: openebs
    resourceVersion: "78191"
    selfLink: /apis/openebs.io/v1alpha1/namespaces/openebs/cstorvolumereplicas/pvc-ffb6210a-dae5-4186-899f-f7f8333d24dd-cstor-sparse-pool-uw8i
    uid: 4131e38d-a31e-4c5d-beb1-d3ab255609d1
  spec:
    capacity: 5G
    replicaid: 9AB518DBF9730BDB258296E3D41178A8
    targetIP: 10.106.163.180
    zvolWorkers: ""
  status:
    capacity:
      totalAllocated: 192M
      used: 275M
    lastTransitionTime: "2020-04-01T09:59:42Z"
    lastUpdateTime: "2020-04-01T09:59:42Z"
    pendingSnapshots:
      istgt_snap2:
        compression: 1.30x
        logicalReferenced: 417277952
        referenced: 319405568
        used: 536576
        written: 173701632
    phase: Rebuilding
    snapshots:
      istgt_snap1:
        compression: 1.52x
        logicalReferenced: 232120832
        referenced: 152257024
        used: 1030656
        written: 152257024
  versionDetails:
    autoUpgrade: false
    desired: 1.8.0
    status:
      current: 1.8.0
      dependentsUpgraded: true
      lastUpdateTime: null
      state: ""

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes openebs/openebs#2936 (Part of the issue will be fixed).

Special notes for your reviewer:

• The test case will be covered in different PR.

Checklist:

• Fixes #
• Labelled this PR & related issue with documentation tag
• PR messages has document related information
• Labelled this PR & related issue with breaking-changes tag
• PR messages has breaking changes related information
• Labelled this PR & related issue with requires-upgrade tag
• PR messages has upgrade related information
• Commit has unit tests
• Commit has integration tests

[vishnuitta]

changes are good..
asked for a fn name change, and few logs

[vishnuitta]

changes are good