Lazy load packages in compat for faster import.

[purple4reina]

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What does this PR do?

Lazy load all import statements in datadog.utils.compat, cutting down time to import datadog from ~64ms to ~34ms.

Description of the Change

In serverless environments like AWS Lambda, the time it takes to import packages is important. On a "cold start" (first invocation on a new container) the caller must wait around for all code to initialize and for their request to be handled.

Customers care a lot about their cold start times in AWS Lambda in particular.

Alternate Designs

Possible Drawbacks

Patching the sys module in this way can be risky. All these imports need to be thoroughly tested.

Verification Process

Before this change, it took 25.6ms to import the datadog.utils.compat module, which is imported every time datadog is imported.

After this change, it now takes 1.19ms to import the same datadog.utls.compat module.

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
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