Increase io_combine_limit range to 1MB.

The default of 128kB is unchanged, but the upper limit is changed from
32 blocks to 128 blocks, unless the operating system's IOV_MAX is too
low.  Some other RDBMSes seem to cap their multi-block buffer pool I/O
around this number, and it seems useful to allow experimentation.

The concrete change is to our definition of PG_IOV_MAX, which provides
the maximum for io_combine_limit and io_max_combine_limit.  It also
affects a couple of other places that work with arrays of struct iovec
or smaller objects on the stack, so we still don't want to use the
system IOV_MAX directly without a clamp: it is not under our control and
likely to be 1024.  128 seems acceptable for our current usage.

For Windows, we can't use real scatter/gather yet, so we continue to
define our own IOV_MAX value of 16 and emulate preadv()/pwritev() with
loops.  Someone would need to research the trade-offs of raising that
number.

NB if trying to see this working: you might temporarily need to hack
BAS_BULKREAD to be bigger, since otherwise the obvious way of "a very
big SELECT" is limited by that for now.

Suggested-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com

src/include/port/pg_iovec.h

@@ -33,8 +33,12 @@ struct iovec
 
 #endif
 
-/* Define a reasonable maximum that is safe to use on the stack. */
-#define PG_IOV_MAX Min(IOV_MAX, 32)
+/*
+ * Define a reasonable maximum that is safe to use on the stack in arrays of
+ * struct iovec and other small types.  The operating system could limit us to
+ * a number as low as 16, but most systems have 1024.
+ */
+#define PG_IOV_MAX Min(IOV_MAX, 128)
 
 /*
  * Like preadv(), but with a prefix to remind us of a side-effect: on Windows