[Issue 440] Reverted datadog to DataDog

[omnilight]

Fixes #440

Motivation

Data Dog has added go.mod file to their lib, and now library is official named github.com/DataDog/zstd
So we must use it to be able to install client correctly

Modifications

go mod change

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)