Proposal: define `--gcttl` of BR as the TTL after backup finished

[pingyu]

Feature Request

Is your feature request related to a problem? Please describe:

The --gcttl is the TTL from the beginning of backup now, and users have to estimate the duration of backup + duration of other preparation before start a changefeed. However, it would be difficult to estimate the duration of backup. Once the duration is underestimated, the initial backup is wasted and must be performed again.

Describe the feature you'd like:

Define --gcttl as the TTL after backup finished. Then users only need to estimate their preparation time before start the changefeed.

Besides, --gctll defaults to 0, which means that user doesn't need a CDC, and holding GC safepoint is not necessary, for saving disk space.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[pingyu]

@haojinming @zeminzhou PTAL~

[github-actions]

This issue is stale because it has been open 30 days with no activity.