Use a single buffer in zstdmt

Use a single buffer in zstdmt

Summary:
Allocate a single input buffer large enough to house each job, as well as
enough space for the IO thread to write 2 extra buffers. One goes in the
`POOL` queue, and one to fill, and then block on a full `POOL` queue.
Since we can't overlap with the prefix, we allocate space for 3 extra
input buffers.

Test Plan:
* CI
* With and without ASAN/UBSAN run zstdmt with different number of threads
  on two large binaries, and verify that their checksums match.
* Test on the tip of the zstdmt ldm integration.

Reviewers: cyan

Differential Revision: https://phabricator.intern.facebook.com/D7284007

Tasks: T25664120