allow jobSize to be as low as 512 KB

[Cyan4973]

answer #2043 .
previous lower limit was 1 MB.

Note : by default, the smallest job size, achieved at level 1, is 2 MB.
So this PR has no impact on multi-threading compression at default settings.
Smaller job sizes can be achieved by manipulating job sizes directly,
or by manually modifying window sizes to lower amounts.

#2043 requested to make the minimum modifiable at runtime,
but it doesn't make sense, since jobSize itself is a runtime parameter.
If there is a lower limit, that's because we believe there are side effects introduced when job sizes are too small, and they should be avoided. The value of the lower limit can be negotiated, but its existence should not.

Minimum job size can still be modified at compile time, setting build variable ZSTDMT_JOBSIZE_MIN.
This makes it practical for tests, in trying to assess what's a better minimum on a specific system.

Also : updated unit test to ensure that this new limit works fine
(test would fail with previous 1 MB limit).

[terrelln]

Looks like this needs to be fixed:

zstd/lib/compress/zstdmt_compress.c

Lines 1243 to 1245 in 12c045f

	U32 const jobSizeMB = (U32)(mtctx->targetSectionSize >> 20);
	U32 const rsyncBits = ZSTD_highbit32(jobSizeMB) + 20;
	assert(jobSizeMB >= 1);

IDK why it converts job size to MB, but it seems like it could just be:

 U32 const rsyncBits = ZSTD_highbit32(mtctx->targetSectionSize);