Lower LZMA memory requirements

[hadess]

LZMA has some pretty high memory requirements when compressing and decompressing. ostree's builtin LZMA compression helpers should take this into account when compressing (maybe only allowing memory usage limitation when running the tests, and not in production).

See the "Memory usage" section of http://linux.die.net/man/1/lzma

[cgwalters]

We may need a way to skip the static delta generation when running tests..."make client-check" or something. But it might also be OK to just limit the requirements and get worse compression.