## Description 

The "first_checkpoint" option is a bit broken.
If first_checkpoint is larger than watermark + 1, for sequential
pipeline this will simply lead to loss of liveness because it always
expects watermark + 1 as the next checkpoint; for concurrent pipeline it
will never be able to update watermark for a similar reason.
This PR adds a check when registering the pipeline to simply not allow
that to happen.

If first_checkpoint is smaller than watermark + 1, the intention must be
that we want to be able to backfill.
However the sequential pipeline will ignore any data that is below the
watermark.
This PR fixes that by still allowing us to commit data even when they
are below watermark.

## Test plan 

CI.
Probably need to add tests too.

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