There is a large buffering effect when using zstdmt in MT mode.
Consequently, data is read first, pushed to workers,
and only later will the compressed result come out.
That means there is no longer immediate correlation
between amount of data read, and amount of data written.
This patch disables the displaying of % compression
when multi-threading is enabled.
It adds the displaying of total size when it can be determined
(it usually can be determined for files, but not for stdin)
so the user has a sense of "how far from the end" the compression compressed is.
There is no modification to decompression side,
since decompression is only single-threaded for now.
In some extraordinary circumstances,
*Length field can be generated from reading a partially uninitialized memory segment.
Data is correctly identified as corrupted later on,
but the read taints some later pointer arithmetic operation.
fileio.c was continually pushing more content without giving a chance to flush compressed one.
It would block the job queue when input data was accumulated too fast (requiring to define many threads).
Fixed : fileio flushes whatever it can after each input attempt.
Sections 2+ read a bit of data from previous section
in order to improve compression ratio.
This also costs some CPU, to reference read data.
Read data is currently fixed to window>>3 size
By default, section sizes are 4x window size.
This new setting allow manual selection of section sizes.
The larger they are, the (slightly) better the compression ratio,
but also the higher the memory allocation cost,
and eventually the lesser the nb of possible threads,
since each section is compressed by a single thread.
It also introduces a prototype to set generic parameters,
ZSTDMT_setMTCtxParameter()
The idea is that it's possible to add enums
to extend the list of parameters that can be set this way.
This is more long-term oriented than a fixed-size struct.
Consider it as a test.
In some (rare) cases, job list could be blocked by a first job still being processed,
while all following ones are completed, waiting to be flushed.
In such case, the current job-table implementation is unable to accept new job.
As a consequence, a call to ZSTDMT_compressStream() can be useless (nothing read, nothing flushed),
with the risk to trigger a busy-wait on the caller side
(needlessly loop over ZSTDMT_compressStream() ).
In such a case, ZSTDMT_compressStream() will block until the first job is completed and ready to flush.
It ensures some forward progress by guaranteeing it will flush at least a part of the completed job.
Energy-wasting busy-wait is avoided.
Like ZSTD_initCStream_usingDict(),
ZSTDMT_initCStream_usingDict() now keep a copy of dict internally.
This way, dict can be released :
it does not longer have to outlive all future compression sessions.
