Add a new compression option for append-optimized tables, "zstd". It is
generally faster than zlib or quicklz, and compresses better. Or at least
it can be faster or compress better, if not both at the same time, by
adjusting the compression level. A major advantage of Zstandard is the wide
range of tuning, to choose the trade-off between compression speed and
ratio.

Update documentation to mention "zstd" alongside "zlib" and "quicklz". More
could be done; all the examples still use zlib or quicklz, for example, and
I think we want to emphasize Zstandard more in the docs, over those other
options. But this is the bare minimum to keep the docs factually correct.

Using the new option requires building the server with the libzstd library.
A new --with-zstd option is added for that. The default is to build without
libzstd, for now, but we should probably change the default to be on, after
we have had a chance to update all the buildfarm machines to have libzstd.

Patch by Ivan Leskin, Dmitriy Pavlov, Anton Chevychalov. Test case, docs
changes, and some minor editorialization by Heikki Linnakangas.