journal/compress: improve xz compression performance

The new lzma2 compression options at the top of compress_blob_xz are
equivalent to using preset "0", exept for using a 1 MiB dictionary
(the same as preset "1"). This makes the memory usage at most 7.5 MiB
in the compressor, and 1 MiB in the decompressor, instead of the
previous 92 MiB in the compressor and 8 MiB in the decompressor.

According to test-compress-benchmark this commit makes XZ compression
20 times faster, with no increase in compressed data size.
Using more realistic test data (an ELF binary rather than repeating
ASCII letters 'a' through 'z' in order) it only provides a factor 10
speedup, and at a cost if a 10% increase in compressed data size.
But that is still a worthwhile trade-off.

According to test-compress-benchmark XZ compression is still 25 times
slower than LZ4, but the compressed data is one eighth the size.
Using more realistic test data XZ compression is only 18 times slower
than LZ4, and the compressed data is only one quarter the size.

$ ./test-compress-benchmark
XZ: compressed & decompressed 2535300963 bytes in 42.30s (57.15MiB/s), mean compresion 99.95%, skipped 3570 bytes
LZ4: compressed & decompressed 2535303543 bytes in 1.60s (1510.60MiB/s), mean compresion 99.60%, skipped 990 bytes

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