feat: add checked socket exhaustion warning when throughput is slow

[kuhe]

Addresses aws/aws-sdk-js-v3#3560

This PR adds a warning:

@smithy/node-http-handler:WARN socket usage at capacity=<X> and <Y> additional 
requests are enqueued. 
See https://docs.aws.amazon.com/sdk-for-javascript/v3/developer-guide/node-configuring-maxsockets.html

Where X is the configured maxSockets for the Node http(s) agent, and Y is the number of open requests.

This warning is emitted in the following situation:

• a request is taking more than socketAcquisitionWarningTimeout ?? connectTimeout + requestTimeout to complete.
  • indicates delay in acquiring a socket
• no similar warning has been emitted in the last 15 seconds.
  • prevent log spam
• the number of active sockets is equal to maxSockets
  • indicates socket saturation
• the number of enqueued requests is greater than or equal to 2x maxSockets
  • indicates socket saturation

Notes:

• it's easy to reach the maxSocket limit even in normal usage. This is why we can't immediately emit the warning on reaching the limit.
• this is why the PR uses a timeout function, and only when there is a backed up queue of requests both in timing and in request count is a warning emitted.

[syall]

Same comment as before, can we mark this with @internal?

[kuhe]

Alternatives are to not let this timeout be configurable, or have a different opt-in to the warning.

Since users are dealing with a mysterious process exit, having the log output without opt-in I think would be better for their debugging experience.

[siebrand]

This causes generalui/s3p#89