Add support for AWS_LAMBDA_MAX_CONCURRENCY

[bmoffatt]

Description of changes:

This change adds support for the AWS_LAMBDA_MAX_CONCURRENCY environment variable. When present, Lambda will handle up to AWS_LAMBDA_MAX_CONCURRENCY events concurrently using a single instance of an execution environment. This environment variable is set by Lambda when a function is configured to use Managed Instances

Notes

• lambdacontext.MaxConcurrency() returns AWS_LAMBDA_MAX_CONCURRENCY, or 1 if not present.
• Concurrent events are only handled when compiling with go version >= v1.22.0.
• A RuntimeAPI client and goroutine are created per unit of requested concurrency.
• If a handler panics (or other internal aws-lambda-go error encountered), the process will still exit. However, before exiting, goroutines blocked waiting for /next will be canceled, and inflight requests will be allowed to run to completion.
• Buffer re-use has been changed to utilize sync.Pool.

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