Add configurable check parallelism control

[pgagnidze]

Currently, updo creates one goroutine per target for concurrent monitoring. While this works well for typical usage, it can be problematic for deployments with hundreds of targets or in resource-constrained environments.

Current behavior

• One goroutine per target (unlimited parallelism)
• All targets checked simultaneously
• No control over concurrent request limits

Use cases for parallelism control

• High-scale deployments: Monitoring hundreds of targets may overwhelm networks or hit rate limits
• Resource-constrained environments: Containers with limited CPU/memory need bounded concurrency
• Rate limiting compliance: Some services require limiting concurrent requests
• Network bandwidth management: Controlling simultaneous outbound connections

Proposed implementation

Add --parallelism N flag and config option:

• Limit concurrent HTTP requests to N
• Maintain individual target timing schedules
• Default to unlimited (current behavior) for backward compatibility

Technical approach

Replace current per-target goroutines with a worker pool pattern that limits concurrent HTTP requests while maintaining individual target timing schedules.