DICT_TF_ZSSIZE_MAX, so that the code can be compiled with a different
uncompressed page size by redefining UNIV_PAGE_SIZE_SHIFT in univ.i.
Currently, the allowed values are 12, 13, or 14 (4k, 8k, 16k).

Make the default compressed page size half the uncompressed page size.
The previous default was 8 kilobytes, which is the same when compiling
with the default 16k uncompressed page size.

rb://50 approved by Pekka Lampio and Sunny Bains.