Currently we allow uploading segments to upsert-enabled table (#6567)

We should extend it to allow uploading segment to any real-time table so that the following features are possible:

• Bootstrap a real-time table
• Back-fill
• Segment merge/rollup
• Segment purge/conversion

With segment upload support, we can remove most of the restrictions to real-time table, and make long retention real-time table much more manageable.