core: replace crc32 implementation

[arogge]

The previous crc32 implementation was not optimal from a performance
point-of-view.
This patch introduces Stephan Brumme's Fast CRC32 which is a lot faster
than the old algorithm.

[arogge]

I'm not sure the files are in the right location. Can somebody please review?

[joergsteffens]

Additional licenses must also be described in https://github.com/bareos/bareos/blob/master/core/LICENSE and https://github.com/bareos/bareos/blob/master/core/debian/copyright.footer

[arogge]

Licensing info will be added once we've finally agreed on the file locations, because the paths to the sources will be coded into the licensing info.

[franku]

Approved, however I would suggest to take only the source code and header files and not the Makefile because we do not use it.

[stbrumme]

Hi, I'm the author of the CRC32 library.
My lib uses the zlib license (see https://github.com/stbrumme/crc32/blob/master/LICENSE ).
Quoting from the license: "an acknowledgment in the product documentation would be appreciated but is not required".

Have a nice day,
Stephan