liblzma: Make lzma_outq usable for threaded decompression too.

Before this commit all output queue buffers were allocated as
a single big allocation. Now each buffer is allocated separately
when needed. Used buffers are cached to avoid reallocation
overhead but the cache will keep only one buffer size at a time.
This should make things work OK in the decompression where most
of the time the buffer sizes will be the same but with some less
common files the buffer sizes may vary.

While this should work fine, it's still a bit preliminary
and may even get reverted if it turns out to be useless for
decompression.