ntdb: reduce transaction pagesize from 64k to 16k.

ntdb: reduce transaction pagesize from 64k to 16k.

The performance numbers for transaction pagesize are indeterminate:
larger pagesizes means a smaller transaction array, and a better
chance of having a contiguous record (more efficient for
ntdb_parse_record and some internal operations inside a transaction).

On the other hand, large pagesize means more I/O even if we change a
few bytes.

But it also controls the multiple by which we will enlarge the file,
and hence the minimum db size.  It's 4k for tdb1, but 16k seems
reasonable in these modern times.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>