[contrib] largeNbDicts bugfix + improvements

[embg]

These are local changes I've been cherry-picking on builds for a while. Figured it's time to put them on GitHub.

• Fixes a bug in the memory estimation found by @terrelln
• Fixes a bug which prevented forceAttach/forceCopy from working properly (ZSTD_compress_usingCDict -> ZSTD_compress2)
• Prints results to a csv file with the executable name
  • The idea is you bake the commit hash and compiler into the executable name. (This is my setup).
  • largeNbDicts will append the best speed to the CSV file, so you can run multiple times and then get statistics. I like to interleave runs between different versions to correct for background load changes.
  • I have a benchmark runner script which creates a directory tree of CSV files for different scenarios, and then a python script which aggregates all the statistics. Will put them on GitHub when I have time :)

[embg]

• Fixed FILE handle leak
• Fixed a bug which prevented forceAttach/forceCopy from working properly (ZSTD_compress_usingCDict -> ZSTD_compress2).