Refactor manifest module

[Rachelint]

Describe This Problem

For current manifest implementation, following problems exist:

• Complexity of snapshot generation, and problem led by giant wal entry Store snapshot of manifest to object store #579
• Pull logs from storage repeatedly while building snapshot.

Proposal

For solving above problem, we plan to refactor manifest module:

• Store the snapshot in oss rather than putting it back to wal storage.
  Store snapshot of manifest to object store #579 by @ShiKaiWi
• Build the snapshot according to data structure in memory rather than pulling logs from storage repeatedly. #801

Additional Context

This is a breaking change #402.