ZSTDMT_compress() creates a single frame

ZSTDMT_compress() creates a single frame

The new strategy involves cutting frame at block level.
The result is a single frame, preserving ZSTD_getDecompressedSize()

As a consequence, bench can now make a full round-trip,
since the result is compatible with ZSTD_decompress().

This strategy will not make it possible to decode the frame with multiple threads
since the exact cut between independent blocks is not known.
MT decoding needs further discussions.