seekable_format/examples/parallel_compression.c is not parallel

[EbiArnie]

Describe the bug
The parallel compression example does not seem to be working in parallel.
It runs on a single thread only, regardless of how many threads are requested.

To Reproduce

1. Build the example parallel_compression
2. Run:
   ./parallel_compression data.txt $((2*1024*1024)) 4
   data.txt can be any sufficiently large file so that compression takes a handful of seconds.
   In my case, this is ASCII data, 2.8 GB.
3. While the compression is running, look at e.g. top
4. Just a single thread is used. This is usually slower than the single threaded seekable_compression,

Expected behavior
Parallel execution should use multiple cores and be faster than single threaded.
The full zstd executable works as expected; given the -T option it is faster than without and saturates all cores.

Also, I'd love for --seekable to become a standard option for zstd, but that should probably be a separate issue.

Desktop:

• OS: Linux
• Version Ubuntu 22.04
• Compiler: gcc 11.4
• Flags: -O3
• System: Quad-core Intel Core i7-1165G7
• Build system Makefile
• Built from git version:
  commit 559762d, Mar 14 2024