[Enhancement] Set Full Snap renewTime to the time full snapshot was taken instead of full snap lease update time.

[anveshreddy18]

Enhancement (What you would like to be added):

During update of full snapshot lease, set renewTime to the time the full snapshot was taken, instead of setting it to the time when the lease was updated as part of retry mechanism.

Motivation (Why is this needed?):

Currently if a full snapshot lease is not updated upon taking a full snapshot, PR #711 has introduced a retry mechanism where in the method heartbeat.FullSnapshotCaseLeaseUpdate that takes care of updating a full snapshot is repeatedly called until the lease is updated. This works. But with this, the renewTime in the lease is set to the time when the lease is updated, ideally this makes sense but druid consumes the renewTime to check the health of backups. So instead of just retrying the method to update the lease, we should carry forward the snapshot time to be set as renewTime even if the lease is updated x hours after the snapshot is taken. This way druid knows only about the snapshot time and need not care about whether it was updated instantly or as part of retry method.

Approach/Hint to the implement solution (optional):