- Factor out LDM's hash function for reuse
- Add rsyncable mode to zstdmt and expose it via the advanced API
- Fix `-B`/`--block-size` to actually set the job size
- Add rsyncable tests to `zstreamtest` and `playTests.sh`

Tested by:

```
> cat A.100MB B.100MB C.100MB D.100MB E.100MB | zstd --rsyncable -fo src/file.zst
/*stdin*\            : 48.22%   (524288000 => 252837782 bytes, src/file.zst)
> rsync -rc --stats src devbigvm:/data/users/terrelln/rsync-test
Total bytes sent: 252868779
total size is 252837782  speedup is 1.00
> echo test > test
> cat test A.100MB test B.100MB test C.100MB test D.100MB test E.100MB | zstd --rsyncable -fo src/file.zst
/*stdin*\            : 48.23%   (524288025 => 252838025 bytes, src/unicorn.tar.zst)
> rsync -rc --stats src devbigvm:/data/users/terrelln/rsync-test
Total bytes sent: 4605696
total size is 252838025  speedup is 53.60
```

Close #1155.

- Add rsyncable mode to multithreaded mode
- Factor out LDM's hash function for reuse

Separating benchfn and benchzstd

because gcc considers that
`void* const *`  and `void**` are 2 different types ...

to use structure for function parameters
as it expresses much clearer than a long list of parameters,
since each parameter can now be named.

with BMK_extract_returnValue()

it shall be possible to use benchfn
without any dependency on zstd.

Fix #1412: Perform Signed Comparison When Setting Attach Dict Param

which is more accurate

Improves decompression speed when using cold dictionary

 improve long-range decoder speed

by triggering the prefetching decoder path
(which used to be dedicated to long-range offsets only).

Figures on my laptop :
no content prefetch : ~300 MB/s (for reference)
full content prefetch : ~325 MB/s (before this patch)
new prefetch path : ~375 MB/s (after this patch)

The benchmark speed is already significant,
but another side-effect is that this version
prefetch less data into memory,
since it only prefetches what's needed, instead of the full dictionary.

This is supposed to help highly active environments
such as active databases,
that can't be properly measured in benchmark environment (too clean).

Also :
fixed the largeNbDict test program
which was working improperly when setting nbBlocks > nbFiles.

Add TSAN dictionary tests

one limit case can apparently be generated during fuzzer tests

as it was broken and no one noticed

on enwik9 at level 22 (which is almost a worst case scenario),
speed improves by +7% on my laptop (415 -> 445 MB/s)

Prevent deadlock on malloc() failure.

Allow creating debug builds with CMake

Only set the build type to Release if the user didn't select any.
See discussion in https://github.com/facebook/zstd/pull/1278#issuecomment-436327923
and below.

fixed T36302471

just add an assert() in ZSTD_insertBtAndGetAllMatches()