Implement the toolkit for managing `Migration`s.

[YuJuncen]

Enhancement

We have designed the migration system for compacting log backup. Generally, every modifications over the "stabilized" storage must be done by appending a "migration".

     +-Modifications over those files                 
     | Must be done by Appending a migration.         
     |                                                
+----|--------------+-------------+                   
| stabilized        | Appending   |                   
|-------------------|-------------+                   
|                    \                                
 \                   |                                
 |Truncated TS        \Resolved TS                    

Migrations are idempotent batch modifications (adding a compaction, delete a file, etc..) to the backup files. You may check the protocol buffer message Migration for more details. Idempontence is important for migrations, as they may be executed multi times due to retry or racing.

[YuJuncen]

We need to implement the toolkit for the Migration storage.