[libzstd] Fix ZSTD_compress2() for multithreaded compression

[libzstd] Fix ZSTD_compress2() for multithreaded compression

`ZSTD_compress2()` wouldn't wait for multithreaded compression to
finish. We didn't find this because ZSTDMT will block when it can
compress all in one go, but it can't do that if it doesn't have enough
output space, or if `ZSTD_c_rsyncable` is enabled.

Since we will already sometimes block when using `ZSTD_e_end`, I've
changed `ZSTD_e_end` and `ZSTD_e_flush` to guarantee maximum forward
progress. This simplifies the API, and helps users avoid the easy bug
that was made in `ZSTD_compress2()`

* Found by the libfuzzer fuzzers.
* Added a test case that catches the problem.
* I will make the fuzzers sometimes allocate less than
  `ZSTD_compressBound()` output space.