Previously, each job would reserve a CCtx right before being posted.
The CCtx would be "part of the job description",
and only released when the job is completed (aka flushed).
For ZSTDMT_compress(), which creates all jobs first and only join at the end,
that meant one CCtx per job.
The nb of jobs used to be == nb of threads,
but since latest modification,
which reduces the size of jobs in order to spread the load of difficult areas,
it also increases the nb of jobs for large sources / small compression level.
This resulted in many more CCtx being created.
In this new version, CCtx are reserved within the worker thread.
It guaranteea there cannot be more CCtx reserved than workers (<= nb threads).
To do that, it required to make the CCtx Pool multi-threading-safe :
it can now be called from multiple threads in parallel.
Makes frame type (zstd,skippable) detection more straighforward.
ZSTD_getFrameHeader set frameContentSize=ZSTD_CONTENTSIZE_UNKNOWN to mean "field not present"
ZSTD_estimateCDictSize() now uses same arguments as ZSTD_createCDict()
ZSTD_estimateCDictSize_advanced() uses same arguments as ZSTD_createCDict_advanced()
ZSTD_estimateCCtx() is now a "simple" function,
taking int compressionLevel as single argument.
ZSTD_estimateCCtx_advanced() takes a CParams argument,
which is both more complete and more complex to generate.
--no-big-tests is typically used in combination with qemu-user-static
qemu-user-static allocated 4 GB of RAM upfront.
On 2 GB VM, this can degenerate into a crash.
It's not a problem as long as memory is not used.
But with multi-threading enabled, memory fragmentation kicks in,
so the amoung of RAM effectively touched increases,
and can pass beyond the 2 GB limit of the VM.
In single-threaded mode, there is no such issue :
memory requirement is smaller, and remains well-located,
so very little fragmentation is expected.
This modification should make `qemu-arm-static zstreamtests --newapi --no-big-tests`
work fine on Travis CI.
