This ticket is to turn on (a degraded version) of hybrid index builds for background, non-unique indexes.

Specifically:

• On a background non-unique index build, install the interceptor that directs client writes to a side-table instead of directly going into the index.
• If trivial, use the external sorter for the table scanning part of the index build. Otherwise, run the hybrid index build in a degraded mode where a bulk insert cursor is not used. The latter case is the expected outcome.
• After the table scan completes, consume the side-table writes adding them back to the real index.
• ??? (make sure the index catalog multikey state is correct)
• Profit. (Complete the index build and remove the interceptor.)