Estimate how long it will take for a rebalance operation to complete

[nbalacha]

There is no way at present to determine when a rebalance operation will complete. This requires admins to keep monitoring the rebalance operation.

The proposed approach will calculate the estimated time every time the rebalance status command is issues. The value will be displayed along with the rebalance status output.

1. Determine the number of files on a brick using statfs
2. Calculate the rate at which files have been processed so far
3. Calculate the time required to complete the operation based on 1 and 2.

This approach will provide a rough estimate of the time required and will be updated every time the status command is issues.

[ShyamsundarR]

This feature is complete and part of master and 3.10 release.

Feature page: https://review.gluster.org/#/c/16119/
Code submission: https://review.gluster.org/#/c/15893/ (master)
https://review.gluster.org/#/c/16458/ (release-3.10 backport)
Release notes: https://review.gluster.org/#/c/16497/