Improve efficiency ratio of incremental chain restore

[gsmolk]

It would be good to avoid block overwriting when restoring incremental chain.

[gsmolk]

First results:

INFO: Backup files are restored. Transfered bytes: 5148MB, time elapsed: 42s
INFO: Approximate restore efficiency ratio: 30% (1554MB/5148MB)

vs

INFO: Backup files are restored. Transfered bytes: 1554MB, time elapsed: 15s
INFO: Approximate restore efficiency ratio: 100% (1554MB/1554MB)

https://gist.github.com/gsmol/1f0e441f6c305b67ea6658217bbca2ec

[gsmolk]

Some more tests:

1. FULL
INFO: Backup files are restored. Transfered bytes: 1554MB, time elapsed: 9s
INFO: Approximate restore efficiency ratio: 100% (1554MB/1554MB)
2. FULL
INFO: Backup files are restored. Transfered bytes: 1554MB, time elapsed: 10s
INFO: Approximate restore efficiency ratio: 100% (1554MB/1554MB)
3. INCR
INFO: Backup files are restored. Transfered bytes: 3525MB, time elapsed: 21s
INFO: Approximate restore efficiency ratio: 44% (1557MB/3525MB)
4. INCR
INFO: Backup files are restored. Transfered bytes: 3525MB, time elapsed: 19s
INFO: Approximate restore efficiency ratio: 44% (1557MB/3525MB)

patched:
1. FULL
INFO: Backup files are restored. Transfered bytes: 1554MB, time elapsed: 9s
INFO: Approximate restore efficiency ratio: 100% (1554MB/1554MB)
2. FULL 
INFO: Backup files are restored. Transfered bytes: 1554MB, time elapsed: 9s
INFO: Approximate restore efficiency ratio: 100% (1554MB/1554MB)
3. INCR
INFO: Backup files are restored. Transfered bytes: 1557MB, time elapsed: 13s
INFO: Approximate restore efficiency ratio: 100% (1557MB/1557MB)
4. INCR
INFO: Backup files are restored. Transfered bytes: 1557MB, time elapsed: 11s
INFO: Approximate restore efficiency ratio: 100% (1557MB/1557MB)