Re: prevent immature WAL streaming

This took some time because backpatching the tests was more work than I
anticipated -- function name changes, operators that don't exist,
definition of the WAL segment size in pg_settings.  I had to remove the
second test in branches 13 and earlier due to lack of LSN+bytes
operator.  Fortunately, the first test (which is not as clean) still
works all the way back.

However, I notice now that the pg_rewind tests reproducibly fail in
branch 14 for reasons I haven't yet understood.  It's strange that no
other branch fails, even when run quite a few times.

-- 
Álvaro Herrera           39°49'30"S 73°17'W  —  https://www.EnterpriseDB.com/
"Siempre hay que alimentar a los dioses, aunque la tierra esté seca" (Orual)