Set ReorderBufferTXN->final_lsn more eagerly

... 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