... specifically, set it incrementally as each individual change is
spilled down to disk.  This way, it is set correctly when the
transaction disappears without trace, ie. without leaving an XACT_ABORT
wal record.  (This happens when the server crashes midway through a
transaction.)

Failing to have final_lsn prevents ReorderBufferRestoreCleanup() from
working, since it needs the final_lsn in order to know the endpoint of
its iteration through spilled files.

Commit df9f682c already tried to fix the problem, but it didn't set
the final_lsn in all cases.  Revert that, since it's no longer needed.

Author: Vignesh C
Reviewed-by: Amit Kapila, Dilip Kumar
Discussion: https://postgr.es/m/CALDaNm2CLk+K9JDwjYST0sPbGg5AQdvhUt0jbKyX_HdAE0jk3A@mail.gmail.com