Add redo snapshotter.

[tejal29]

The Redo snapshot mode, use the following heuristics to calculate if a file has changed

1. File Mod Time
2. File Size
3. File Mode
4. File Group Id, UserId

This is inspired from https://apenwarr.ca/log/20181113
Redo snapshotter has proved to increase the performance for large file systems.
The performance could be upto 56%
See more at graph
As the number of files increase, time spent in hashing increases

[tejal29]

Performance Improvements

Number of Files | Total Build Time Full Snapshot - current | Total Build Time Redo Snapshot | % decrease
-- | -- | -- | --
10000 | 13.834973 | 14.722817 | -6.42%
50000 | 47.926338 | 45.414848 | 5.24%
100000 | 96.440554 | 91.042636 | 5.60%
200000 | 187.346273 | 179.147903 | 4.38%
300000 | 341.233622 | 263.565265 | 22.76%
500000 | 1404.410121 | 562.507168 | 59.95%
700000 | 1772.379601 | 763.790115 | 56.91%