Tracking issue of unifying duplicate packages between BR and Lightning

[overvenus]

After #665, BR repo contains both BR code and TiDB-Lightning code.

There are some packages are very similar, they should be unified to reduce duplicate code and improve maintenance efficiency.

• 1. pkg/glue and pkg/lightning/glue
• 2. pkg/logutil and pkg/lightning/log
• 3. pkg/checksum, pkg/lightning/restore/checksum.go and pkg/lightning/verification
• 4. pkg/kv and pkg/lightning/backend
• 5. pkg/httputil and pkg/lightning/common/security.go
• 6. pkg/utils/worker.go and pkg/lightning/worker
• 7. pkg/manual and pkg/lightning/manual
• 8. pkg/utils/version.go and pkg/lightning/common/version.go *: unify version and set max compatible via NextMajorVersion #798