Configurable compression level for archive storage engine

[gc-code]

I have added a system variable that allows the user to configure the compression level used by the archive storage engine. This was listed as one of Brian's TODOs and I think it would make a nice addition to the engine. Find below a comparison of the ARZ file sizes for different values of 'archive_compression_level' generated by the windows 'dir' command:

05/08/2018  12:04         4,259,928 10_employees.ARZ     :: Compression level 10 (max)
05/08/2018  12:18         4,816,984 0_employees.ARZ      :: Zlib default compression
05/08/2018  12:12         9,076,827 1_employees.ARZ      :: No compression

The prefixed number is the compression level. I have shifted Zlib's default compression levels up by one because the variable list doesn't support signed integers. For example, the default compression level is now 0 instead of -1.

I intend on continuing to work on TODOs around the code if you like what I have done here. Constructive feedback is appreciated.

Thanks,
George

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[gc-code]

OCA submitted

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[gc-code]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

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