compression: add build-time option to select default

compression: add build-time option to select default

Compression and decompression are controlled by the same build flag,
so if one wants to use, say, LZ4 to compress, ZSTD has to be disabled,
which means one loses the ability to read zstd-compressed journals.

Add a default-compression meson option, that allows to select any of
the available compression algorithms as the default.