drainer OOM in old cluster

[YuJuncen]

Bug Report

Please answer these questions before submitting your issue. Thanks!

From AskTUG: https://asktug.com/t/topic/274497

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   Start drainer in a cluster with long history.

2. What did you expect to see?
   Drainer can startup (even slowly).

3. What did you see instead?
   OOM. The peak memory usage is up to 64G. Note that the GlobalID in the cluster is 4593562(which means there probably are millions of DDL jobs).

4. Please provide the relate downstream type and version of drainer.
   (run drainer -V in terminal to get drainer's version)
   v5.1.2

5. Notes
   As a solution, we can use the CDC like method to resolve this: loading the snapshot and then send DDL jobs incrementally.
   Or, we can paginate the scan of history DDL jobs for reducing the peak memory usage.

[lichunzhu]

Possible solution:
Use CDC's snapshot method and just load the following jobs. https://github.com/pingcap/tiflow/blob/c063509/cdc/entry/schema_storage.go#L728

[kennytm]

already "fixed" in #1237 i believe?