make `allow_ingest_behind` option dynamic

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### Expected behavior

While `allow_ingest_behind` is enabled, features are disabled like seqnum zeroing, compaction to bottom level (universal or `CompactRange()`), and hopefully soon MERGE->PUT conversion (#6490). The user should be able to dynamically disable `allow_ingest_behind` to enable these features after ingestion completes.

### Actual behavior

The user currently must restart the DB after ingesting-behind in order to enable the above-mentioned features. That's because `allow_ingest_behind` is not a dynamic option.