RUM-3574 perf: Start sending batches immediately after feature is initialized

[ncreated]

What and why?

📦⏱️ This PR removes the initial delay for sending batches. Now, first upload will be triggered soon after feature is initialized.

The motivation is to maximise a chance of sending data if application is impacted by early crash.
Related to DataDog/dd-sdk-flutter#573

How?

Removing initial delay from DataUploadWorker.

Before:

----[I]------------[U1]----------------[U2]-->
     ^ SDK init
                    ^ initial upload
                                         ^ next upload

Now:

----[I][U1]----------------[U2]-->
     ^ SDK init
        ^ initial upload
                             ^ next upload

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes

Custom CI job configuration (optional)

• Run unit tests for Core, RUM, Trace, Logs, CR and WVT
• Run unit tests for Session Replay
• Run integration tests
• Run smoke tests
• Run tests for tools/

[datadog-datadog-prod-us1]

Datadog Report

Branch report: ncreated/RUM-3574/start-sending-batches-on-sdk-init
Commit report: 1f60725
Test service: dd-sdk-ios

✅ 0 Failed, 2767 Passed, 0 Skipped, 6m 34.99s Wall Time
🔻 Test Sessions change in coverage: 8 decreased, 5 increased

🔻 Code Coverage Decreases vs Default Branch (8)

This report shows up to 5 code coverage decreases.

• test DatadogCoreTests tvOS 79.31% (-0.31%) - Details
• test DatadogLogsTests tvOS 45.09% (-0.28%) - Details
• test DatadogLogsTests iOS 45.04% (-0.28%) - Details
• test DatadogCrashReportingTests iOS 27.68% (-0.2%) - Details
• test DatadogRUMTests tvOS 80.83% (-0.2%) - Details

[ncreated]

Back to draft as the CI got flaky on BG tasks tests in DataUploadWorker.

[ganeshnj]

I'm curious about the impact when a notification triggers app load and we will send anything pending all at the same time.

[ncreated]

I'm curious about the impact when a notification triggers app load and we will send anything pending all at the same time.

As we discussed, we only change the delay of initial upload, so practically there won't be a change to the load spike in this scenario - it will be only shifted earlier in time.

[maxep]

👍